The idea of using grub as a replacement sounds interesting. I played around a bit trying to install grub from the gentoo-based systremrescuecd, but ran into some error while trying to update the chainloading code. I'll see if can look into it more thoroughly over the next few days.
I'm still convinced it's just an error in the bootloader code as the FreeBSD image boots without a problem, furthermore OpenBSD's code was originally imported from FreeBSD as the version control headers clearly show
http://cvsweb.openbsd.org/cgi-bin/cv...d/efi/include/