I've spent about 10 hours searching the codex, looking at source, and peeking at other plugin source files.
It would appear that there is no
apply_filters hook for changing the query string so that a plugin could be written to search multiple "posts" tables other than the current blog. I can change the "where" portion of the query, but not what is actually searched (as near as I can tell). Such a hook would be useful, I think, for those of us who run multiple blogs out of a single database.
I think I've read that the developers are working on revamping the searching, so perhaps this is something they're considering already.