I simultaneously accelerated the passage of time and decreased its velocity by sleeping through it.
Hoping that despite the boot errors OpenBSD will work with this nice new drive, I applied all the 2.6 patches, built a custom kernel with SCSIDEBUG and FFS_SOFTUPDATES defined, updated Apache to 1.3.11 from -current, and built MySQL from the -current ports tree. Then, just for comparison, I test-booted a NetBSD 1.4.1 GENERIC kernel with no errors at all. Interesting. Reported this finding to the OpenBSD/mac68k mailing list in the hopes of getting some expert advice.
All of this is not very interesting to the visitor who just wants to see the site and can't. I'm trying to keep the informational placeholder page (on my firewall) up to date with my progress on restoring the site to service.