Seems to me that the benefit of chamber sleeving 7.62 down to .22 would be about the same as dry firing. The grouping of the .22 rounds would be sloppy at best (the bullet wouldn't be engaging the rifling - just rattling its way down the barrel).

If it's a barrel & chamber sleeving then ... maybe. (is that even done?) A conversion kit would make the most sense to me.

My experience as a Range Officer for an indoor pistol range is that people need lots of .22 practice (myself included) to keep from developing a flinch or to beat it back down when it develops. You can shoot a LOT of .22 for the cost of most centerfire ammo.