Consider using molten metal instead!
Most of the problems facing your lava waterfall can be avoided by using a more easily workable material. Relative to lava, almost all molten metals emit more heat and light than lava at a given temperature as you can see from the chart below. (stainless steel, tungsten, copper, aluminium, etc. almost all metals will follow this pattern)
In addition to aesthetics, metals can be heated using induction which is much more efficient and compact than whatever mechanism would heat your lava. This process also makes no direct contact with the metal in question, so it will not wear out as easily.
By using electromagnetic induction from a high-energy coil, the molten metal's temperature can be very accurately controlled and will also cool more quickly than stone (meaning the transport system needed to recycle it will be much easier to create and maintain).
Let's suppose you were to use a molten metal like tin or pure iron (avoid steel due to the sparks of super heated carbon that jump out). It can be made to flow much more easily (and better resemble a waterfall) than lava. It is highly receptive to inductive heating. It releases none of the toxic fumes that true lava would, and unlike lava, it can even be moved around using a ceramic pump. Furthermore, molten metal can be more easily contained, guided, split, or pushed out of the way to reveal your door due to its lower viscosity at higher temperatures.
Suppose the following for your villainous lair!
your passageway to the uber-secret room is disguised as the idealized lava wall you desire, radiating heat and light to whatever specifications you wish. The flow of metal falls cleanly from a slot in the ceiling into a tungsten grate in the floor that drains it away into a reservoir by a trough. the metal cools during this process, becoming easier to transport, but not solidifying. It is pumped through a series of tubes that have inductive coils around them as shown which sustain the metal at this temperature:
From there, the molten metal is taken back up into the ceiling where a series of final high-energy coils return it to the yellow glowing radiance you desire. It is then flowed back over the lip of the ceiling, completing the loop.
In order to reveal the secret massage, one need only turn off the pump, or one could divert the flow to stop only the middle of the lava wall. One need only cross the glowing tungsten grating (perfectly safe if done quickly enough) and behind the wall is a hallway or catwalk or door to your lair.
Edit: also consider this design used by foundries to improve laminar (smooth) flow and prevent strain on the pump