Patches welcome. The first step would be iconv in OpenBSD base (there are existing BSD-licensed implementations to start from). Then have mg make use of it. http://marc.info/?l=openbsd-tech&m=134581162518146&w=2
(Edit: Or try adding a pure UTF-8 mode)
Lacking the time at the moment, but I've been meaning to hone my C skills and Hacking on OpenBSD seems like the best idea. Hopefully, I can finish this M.Sc. soon :)