P.S. Not to start a holywar, but FYI: in a similar project where we used MySQL now we have about 6000 DBs and everything works like a charm.
You seem to have answered your own question here. If my recollection of a previous discussion about many schemas and pg_dump performance is accurate, I suspect you are going to be told that you've got a data architecture that is fairly incompatible with postgresql's architecture and you've specifically ruled out a solution that would play to postgresql's strengths.
