It seems at one point it was supported, I'm seeing references on the QEMU mailing lists for patches as late as August 4th for fixing USB host on BSD.
Sorry, I wasn't aware there was a backend on BSD, it was silently disabled by one of the QEMU developers upstream in 2009.
http://lists.gnu.org/archive/html/qe.../msg00692.html
You should bring this up on the OpenBSD ports@ mailing list, or mail the port maintainer and see if he will add this for you.