I follow the published data from Accurate powder to load 75 grain subsonics. Unfortunately, the powder from this load data has been discontinued, and as a result they have removed the load from their latest version of their data.
I spoke with one of their technical folks at the NRA convention, and he said that they would be more than happy to come up with new subsonic .223 load data with a different powder for anyone emailing them a request.
I have had no trouble with stability using Prvi 75gr projectiles in my 1/7 twist Colt and LWRC barrels, but I would thoroughly test any new load before placing your expensive can in harms way from a baffle strike.
As to the .22lr suggestion, I use my .22lr upper suppressed all the time, and it is quite fun, but the 75gr .223 is quieter because it does not work the action. With 40gr subsonic .22lr at 1050fps and 75gr .223 at 1050fps, the .223 has exactly 1.875 times the energy, and better downrange trajectory due to a higher BC. On the down side, CCI subsonic .22lr will expand consistently from a rifle and can be shot rapid-fire, whereas a 75gr subsonic .223 will never expand when subsonic, and turns your AR into a straight-pull bolt action.