I took the distributed image from the ftp.freebsd.org site, and had to get bootcode.bin and start.elf from the current Raspbian distribution in order to get it to boot. Then I had to repeatedly power-cycle until I got a good boot so I could update the /boot/loader.conf file - now it boots flawlessly.
However. I only see a fraction of the 512MB RAM (off the top of my head, 128MB - could be 256MB, but I'm at work now and don't remember). I tried a couple of different values for gpu_mem and didn't seem to affect this.
Operator error. I wasn't downloading the firmware from github, I was downloading some kind of HTML.
With that error corrected I'm seeing all 512MB (or most of it anyway), and building packages is going much better.
Recap: download RPI-B image here
, update firmware files from here
(i.e. bootcode.bin and start.elf), all set.