plugin hook for tables to query

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.