You're correct, my live media is out of date.
To be clear, the boot process used CD9660 filesystems (one or two, depending upon the particular system) and loaded six filesystems into RAM: /etc, /var, /tmp, /dev, /root, and /home. It used
mfs(8) for these.
I'd begun looking at mass storage based live media, but ended up abandoning this due to the impossibility of integrating these with optical configurations and the inability to "fit" a reasonable toolset into a small USB device. That was several years ago; these days USB devices are larger and I may revisit this.
The only major change I would make is to replace mfs(8) with
mount_tmpfs(8), which will be available with OpenBSD 5.5.
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The only boot-to-RAM OpenBSD system that I'm aware of is
bsd.rd, the RAMDISK kernel used for install, upgrade, and rescue, since it does not depend on any backing store. It uses
rd(4) rather than mfs(8). This requires the filesystem to be located inside the kernel image.