Well, I was not talking about
after you have the drawings. Negotiate this up front, long before drawings change hands.
If you were consulted to build a steel widget and your client agreed to supply the parts, but all they sent you was plastic parts. You would either go back and fix this problem at the source, or learn to make steel out of plastic.
I know this isn't addressing your original request. Sorry.
What you want to do can be done, but you'll need a recursive function to iterate the blocks collection down to the bottom xref in each tree of parent xrefs.
Nested.lsp (available at
http://www.hyperpics.com/customization/autolisp/autolisp_downloads.asp , © Lee Ambrosius) looks like a good starting point. I was able to take this code and extract the INS PT for a series of nested Xrefs. From there, just save this list, detach them all, and reattach.