MySQL is just too ugly of a database. If a daemon based rdbms I'd go for postgres.
Probably you could write an SQL backend in such a way that it works for SQLite, MySQL and PostreSQL at the same time, so this should not be a major issue.
I've considerd this but I think that it will complicate the code quite a bit. Yes each of these are SQL servers but the details of how they do things is different. SQLite for example does not have data types. Its all dynamic. I feel the cleanest and most maintaible method is seperate databases for each server type.