prioritizing aggregated DBs

Hey all, I've been using ->add_database for a few years
to tie sharded DBs together and it works great.

Now, I want to be able to search across several DBs
which aren't sharded, say: linux-DB, glibc-DB, freebsd-DB.

I want to search for something across all of them, but
prioritize results to favor one or some of those DBs over
others.  Is there a way to do that without reindexing?

Or would I fiddle with wdf_inc for all ->index_text and
->add_term calls on a per-DB basis?

Thanks.