Hi Mike,
There should be no need for a filler. The pan should fit snug all the way round. If the plate is filed and polished too much that fit could get sloppy but there is no need to fill it. When polishing a Siler lock, or any with a detachable pan, either polish it with the pan in place or do so very carefully if removed. The collar can be peened down if desired but there is no reason it has to be sealed with anything.
NHGrants, many Germanic and some French locks had detachable pans. It makes it easier to forge the lock plate. You could certainly solder it in place but there is no need.
dave