I thought it would work because I had sub-partitioned that 15GB under the OpenBSD installer so that the first OpenBSD partition was well within that limit, but it didn't work. The trick is that the entire partition as seen by Disk Utility on OS X must reside in that 8GB playground. What I had to do was create two "Free Space" partitions as placeholders in OS X, the first of which fit entirely into the 8GB limit, and a second which was as big as I liked into the lower part of the drive. Then under OpenBSD, I further partitioned that first placeholder into the / and swap partitions, and the second placedholder into the /usr, /tmp and /home partitions.
The last catch was that I could not get it to boot off the hard drive by copying the file ofwboot to the first HFS partition as described in the documentation, so I left the OpenBSD installation CD in the drive and used the Open Firmware command boot cd:,ofwboot /bsd to get it all going.

