Supressors only act on the muzzle blast noise. They don't do anything about the supersonic crack due to the bullet breaking the sound barrier. You need to use sub-sonic loads in conjunction with a suppressor to get reduction of all the noise sources involved. You see suppressors used with supersonic rounds because just cutting muzzle blast reduces sound significantly at the shooter's end, and for military snipers it makes it difficult to tell where a shot was fired from. It just fails to prevent a missed target from realizing he's being shot at, because the supersonic bullet sounds about like a .22 rimfire going off as it passes by.
Subsonic loads fired using small quantities of fast powders, like we are discussing, will lack the supersonic crack, and, because the total gas volume generated isn't large, the muzzle blast, though not suppressed, will be a lot more like the aforementioned .22 rimfire loads than a full power .223 load.
If you can fire a jacketed pistol bullet at subsonic velocities, there is no reason you can't fire a rifle bullet at those velocities. It's just ratios of pressures and bore diameters and barrel lengths and gas volumes. Barrel friction becomes a more significant actor as loads get lower in pressure, so it sometimes takes a bit more powder than you expect. I would not try to load one down very much below the speed of sound in a rifle barrel, just because you don't want a stuck bullet. For that reason, subsonic loads are usually worked down rather than up. You take a load like IMR's recommended minimum charge of Trail Boss, which is 70% fill of the space under the bullet for jacketed rifle bullets. Measure the velocity, then work down to just below the sound barrier if it isn't there already.