I got Wakaba working with PostgreSQL. It took a little bit of fiddling with the table structure, but the changes are worthwhile on the whole, I believe:

• changed IP and mask fields to integers. (Why were they stored as TEXT in the first place?)
• moved static IP mask calculation from SQL into Perl
• all INSERT statements explicitly state their values

There's still a few more things that should be done, particularly rewriting the COUNT(*) queries as Postgres doesn't even try to optimize them, and modifying the table init check to convert the data from an older version. It's a start though, and everything seems to function.