I'm not an advanced lisper so I hate to say "this seems like it should be easy" - it isn't for me. But while playing around with the default archive behavior (C-c C-x C-a OR C-c C-x C-s?), I notice that ":ARCHIVE_OLPATH: Testing/SomeTest Archive Project" is include in the archived item. So Org appears to know where the item should go (if not at the top level). It seems like it should be easy to either a) put it in the right place when archiving, b) put it in the right place at some later point in time or c) perform a search on the archive file that at least limits the results to a tree or subtree. I can do the last one!
-k.