No, this would not work.
The disc between the propellant and light gas can't be driven faster than a bullet would by the same propellant, and it can't in turn compress the gas any faster than it is itself traveling.
In order to accelerate the gas, you need to compress it further by forcing it into a funnel, as described by Wikipedia:
One particular light-gas gun used by NASA uses a modified 40mm cannon for power. The cannon uses gunpowder to propel a plastic (usually HDPE) piston down the cannon barrel, which is filled with high-pressure hydrogen gas. At the end of the cannon barrel is a conical section, leading down to the 5-mm barrel that fires the projectile. In this conical section is a stainless steel disk, approximately 2 mm thick, with an "x" pattern scored into the surface in the middle. When the hydrogen develops sufficient pressure to burst the scored section of the disk, the hydrogen flows through the hole and accelerates the projectile to a velocity of 6 km/s (22,000 km/h) in a distance of about a meter.
In order for this to work, your rifle would need such a piston arrangement, a mechanism for replacing burst disks, and a substantial supply of hydrogen - more than would be able to fit in a conventional cartridge. It would need separate feed mechanisms to supply propellant at the breech and bullets midway down the barrel. It would be a huge amount of added work, added complexity, and added weight. This wouldn't be a magazine of special ammo you could issue to an ordinary rifleman, it would be a whole heavy weapon in its own right - and frankly it's hard to see what advantages it has over a conventional anti-materiel rifle at that point.
Your question talks about maximizing muzzle velocity, but muzzle velocity isn't the main factor you're concerned with in the anti-armor role. (In air-defense and especially missile-defense contexts, it can be more important because your time to intercept is so short, every fraction of a second counts. But a tank will generally still be there if your bullet takes a split second longer to reach it.) What matters is kinetic energy, which is a function of velocity and mass. Chasing expensive gains in muzzle velocity may not get the same bang for your buck as simply getting a bigger gun.