at subsonic velocities, you do not need a gas check. they only are needed when you try to push the pressure envelope, or have a poor fitting bullet
as for jamming up a silencer? I do not personally have experience with one, but I do have interest in the concept, and have read a lot on it.
You will get no buildup of lead, cause the bullets do not touch the silencer.
as for lube, you may get some build up with that depending on the lube itself. but as you are subsonic, it doesn't take much of a lube to perform well. Contact White Label Lubes, and ask your questions there.
or you can try the LLA cut with MS. most folks put it on too thick. It should dry Non-tacky.
or search out "Recluse Tumble Lube" using LLA, Johnson Paste wax, and mineral spirits. drys hard and smooth. should work well in your application