The article On the Potential of Silicon as a Building Block for Life, published just last year, while not directly answering this question, provides a lot of useful information for making progress on it!
In particular, it turns out that silicon actually has more diverse stable chemistry in sulfuric acid than it does in aqueous solutions (although not as many as it does in aprotic solvents), to an extent which more than makes up for the reduced options for carbon chemistry--as long as both carbon and silicon are available as backbone atoms. Thus, a variety of Si-O (siloxane), Si-C, Si-Si, and C-C bonds are expected in different functional positions.
Contrary to the expectation that a more highly oxidizing environment would favor more highly oxidized biomolecules, though, too many hydroxide groups ends up making stuff unstable against hydrolysis (which is why glucose decomposes in sulfuric acid), so we can expect actually proportionally more oxygen released in photosynthesis than occurs on Earth in water-based photosynthesis, and more methyl and silyl groups instead. And silicon double bonds are sufficiently unstable that they are just as unlikely to occur in sulfuric acid chemistry as they are in aqueous chemistry.
This leads me to the conclusion that something like hexamethylcyclotrisiloxane (C6 H18 Si3 O3) could be a reasonable analog to glucose in the sulfuric acid system.
Despite the chemical versatility of organosilanes and organosiloxanes in sulfuric acid solution, however, there is a major barrier: inaccessibility of silicon trapped in rocks as SiO2. Silica is not particularly soluble in (most) acidic conditions--but, it is much more soluble in a mixture of hydrofluoric and sulfuric acid, and it turns out that fluoride and chloride salts are unstable in sulfuric acid as well--meaning that sulfuric acid seas will result in fluorine being extract from crustal rocks and turned into exactly the hydrofluoric acid we need! Silicon tetraflouride, on the other hand, despite being unstable in aqueous solution, is stable in sulfuric acid, and is more energetically favorable than silica.
This leads to a multi-stepped carbon-and-silicon fixation cycle.
The high-level formulation is as follows:
12 CO2 + 6 SiO2 + 18 H2SO4 -> 2 C6-H18-Si3-O3 + 18 SO3 + 51 O2
where SO3 is partially dissolved into the sulfuric acid background and partially released into the atmosphere along with free oxygen.
Slightly more specific steps are as follows:
Silicon Fixation: SiO2 + 4 HF + 2 SO3 -> SiF4 + 2 H2SO4
(The silicon fluoride generated here can go into multiple synthesis paths forming more complex biomolecules with fluorine heteroatoms, but we'll focus on the most basic glucose-photosynthesis functional equivalent.)
Carbon Fixation: 2 CO2 + 4 H2SO4 -> 2 (CH3)HSO4 [methyl sulfate] + 2 SO3 + 3 O2
Those two processes feed H2SO4, and SO3 into each other, and when combined simplify to
Mixed Fixation: SiO2 + 4 HF + 2 CO2 + 2 H2SO4 -> SiF4 + 2 (CH3)HSO4 [methyl sulfate] + 3 O2
Those two methylsulfates and one silicon tetrafluoride can then be combined as follows:
Silicon methylation: SiF4 + 2 (CH3)HSO4 -> (CH3)2SiOF2 + H2SO4 + SO3
Note that the retention of terminal fluorines is necessary to prevent spontaneous hydrolysis, leading to the formation of polydimethylsiloxane. Triplets of those dimethyldifluorosilanol units can then be combined as follows:
6 (CH3)2SiOF2 + 6 H2SO4 -> 2 (CH3)6-(SiO)3 + 12 HF + 6 SO3 + 3 O2
Throughout the process, hydrofluoric acid is regenerated, and sulfuric acid is consumed to donate hydrogens for fixation and release sulfur trioxide into the seas and atmosphere, and oxygen into the atmosphere.
Respiration can use either SO3 (producing SO2 which will later combine with O2 anyway) or O2 directly, in either case producing water which will combine with ambient SO3 to regenerate sulfuric acid, carbon dioxide, and silicon dioxide again. Some intentional silicate deposition may sometimes be intended (such as Earthling plants use to beef up their cell walls, or diatoms use to construct their shells--or as silica-sulfuric acid to use a catalyst for other reactions), but in general, just as primary producers must actively use hydrofluoric acid to extract and fix silicon in the first place, complex multicellular beings will need to actively concentrate HF in their bodies to maintain solubility of silica until it can be excreted. So, urine will be a valuable fertilizer, not just for its content of fixed nitrogen as on Earth, but for its concentration of HF making silica more easily fix-able. Alternatively, respiration just might recruit HF to produce exhalable SiF4 and more water/sulfuric acid rather than letting it go all the way to silica (except where silica deposition is desirable). In that case, we'd end up with a silicon cycle somewhat analogous to the nitrogen cycle on Earth, where microorganisms work hard to fix silicon into a biologically-usable form as SiF4, that gets used by other producers to build biomass, animal-equivalents excrete waste silicon as Si4 to avoid growing "kidney stones", and other microorganisms ("desilifiers" rather than "denitrifiers") react sulfuric acid with SiF4 to regenerate HF and silica.
Now, I don't know for sure that hexamethylcyclotrisiloxane is necessarily the best glucose-analog, or that these specific (still very sketchy) synthesis steps are the best way to get there, but I think this provides a decent proof of concept which indicates that:
- Organisms of some sort, whether they are also the photosynthesizers or symbiotic microbes like Earth's nitrogen-fixers, will use HF to catalyze fixation of silicon.
- Life forms in general will actively concentrate HF in their bodily fluids above the background levels in the seas, and may try to maintain silicon in fixed form throughout their metabolism (just like we don't breathe out N2 as a byproduct of protein metabolism, but rather excrete urea).
And furthermore, an ecologically-equilibrated atmosphere will probably have a mix of carbon dioxide, sulfur trioxide, oxygen, hydrofluoric acid, and silicon tetrafluoride gasses (along with the inert components like nitrogen), with animals breathing in both sulfur trioxide and oxygen and breathing out both CO2 and SiF4, plants doing (on average) the opposite, and microbes cycling silicon and fluorine back and forth between silica and HF vs. SiF4 and hydrofluoric acid.
EDIT:
Supposing that SiH4 is not actually a convenient form to fix silicon in, it is also conceivable that respiration in multicellular organisms could halt at something like disiloxane [(SiH3)2O], which is also conveniently gaseous. Unfortunately, I can't find any information on how disiloxane reacts in sulfuric acid; all of the accessible literature is on fully methylated organosiloxanes, and methyl groups are precisely what you would want to burn off in aerobic respiration!
In this case, disiloxane gas would be available to plants rather than SiF4. This could be hydrolyzed to produce Silyl hydrogen sulfate as follows:
(SiH3)2O + 2 H2SO4 -> 2(SiH3)HSO4 + H2O
(So maybe that's the basic fixed form of silicon, remaining in solution just like nitrogen compounds in Earth biology?)
That makes the basic fixed forms of silicon and carbon entirely parallel.
Combining them to produce hexamethylcyclotrisiloxane then ends up re-consuming some of the oxygen produced during fixation and regenerating a bunch of sulfuric acid:
6 (SiH3)HSO4 + 12 (CH3)HSO4 + 3 O2 -> 2 (CH3)6(SiO)3 + 18 H2SO4
And the silicon fixation reaction ends up replaced with this, parallel to carbon fixation:
2 SiO2 + 4 H2SO4 -> 2 (SiH3)HSO4 + 2 SO3 + 3 O2
And "desilifying bacteria" would simply perform that same reaction in reverse, and/or oxidize disiloxane.
