Maybe my reply didn't make it: use ports' mail/nail package. It did.
Nail is one option, but seems a bit much for the want of a single option, when the program that ships with OpenBSD will already do. It will be easier (and perhaps a learning experience) to submit a patch to the current Mail, or decide that a signature is something non-standard (or not-yet-standard), and therefore not worth the bother.
