For what its worth, I had a flea market user # 60 knuckle cap block plane 
with the same worn out cap screw hole problem. I took a center punch and 
gently punch several spots around the hole to "shrink" the wear areas to a 
smaller diameter, that seemed to do the job for mine. The bolt looked okay 
and it would hold if driven further into the plane body. I think the top two 
threads just wore down to the point of  failure.
 I think you would have to dress the frog a bit after this modification, but 
if it is just for a user, no one would see any difference unless 
disassembled.

 Dave Chatham,  successfully lurking since 2002
(I guess now it is unsuccessfully) 

------------------------------------------------------------------------