@nruddock

The problem seems to have been the welding, as I suspected. I stripped the weld statements out of the target figure, and now it seems to  work ok no mater what geometry I inject.

So the problem seems to be Poser struggling to weld things that were never meant to welded together. When I used morphed obj files for the thighs spawned from the the figures obj file, Poser handled it ok, even though the edge vertices where the weld happens had been moved. When I used box props that came in at a totally diffrent position (on the ground) and had a totally difrent shape Poser threw a fit. When I used thighs from diffrent figures that matched the shape and position more closely I got intermediate results, Poser did not like it, but still worked to some degree.

So in conclusion, it seems to be ok to use a morphed version of the original geometry, even if its edges do not meet the other parts of the figure, but a totally forign object causes problems (unless 'bend' is turned off).