I very much doubt that just marking a partition as OpenBSD is the end of the architecture specific stuff. Somehow the firmware in your computer has to actually boot the OS. Unless things are setup in a way which is acceptable to the firmware with regards to booting (be it mandatory filesystems, directory structure and file formats, master boot records, partition table entries and corresponding flags, GUIDs, bootsectors, etc.), nothing much is going to happen when actually trying to boot it.
Really. Once the OpenBSD partition boot sector gets control everything is done the OpenBSD way. There is some compatibility stuff for PC type partitions but it does not affect normal operation in any way. The GUIDs are the OpenBSD GUIDs. You don't even need a GPT for disks larger than 2GB because the OpenBSD stuff works up to 64PB.
