Seems to work in P6 too!

With the benifit of hindsight, refreshing the welds makes sense. I assume that poser is keeping some data relating to the original welds cached, and trying to apply it to the new geometry. Redoing the welds forces poser to refresh the cache with data relating to the new geometry. This explination is of course just a guess, and may be wrong. The one thing that surprises me a little, is that this data seems to relate to the whole butock geom not just the touching edge vertices.

Some quick tests I did on V3RR in P6, seem to confirm that your method works, though I am a bit surprised that you don't seem to need to re-weld the child (thighs) of the injected part. The testes I did  injected box prop geometry via 'objFileGeom' and P4 thighs via 'geomCustom'.

Welds in the restore poses have not seemed necessary in my tests in P6, but perhaps it is best to include them just to be on the safe side.