Found it!

I was >this< close to writing my own damn set of Perl scripts to test a firewall, but I decided to search one last time. Good thing, too: ftester looks pretty close to perfect.

I'm having trouble right now getting ftestd to work on an OpenBSD 4.3 system; this may be because I'm trying to get it to listen on an interface that's part of a bridge. I'll have to look into this further. But testing it out between my laptop and desktop works a treat, whether my laptop is running OpenBSD or Linux 2.6. Sweet!