The disklabel's sector location on a drive (or, if there is an MBR partition, within the MBR partition) is architecture specific, defined in /usr/include/machine/disklabel.h
While I think there should be "room" in the first sectors, the first track is reserved for both the MBR and for additional boot blocks, so you may be better off adding a partition entry for OpenBSD -- I'd recommended a megabyte, or, perhaps you can reserve just the second logical track.
The goal is to avoid overlaying anything important: either a boot block, or the disklabel.
