Hi Slobodan,
Firstly, I very much doubt Postmodern will be faster for fixed-length fields. I doesn't special-case them at all, so they are sent in the exact same way as other strings. And even if they were, there is no reason they'd be faster -- the Postgres protocol null-terminates every string.
But if you need fixed-width fields, you can probably add a initialize-instance :after method to your DAO class that sanitizes the strings.
Best, Marijn