ok, so I processed the update on the dev server.
I upgraded FreePBX from 10.13.66-8 to the current 10.13.66-11 using the sequential update scripts at
http://wiki.freepbx.org/display/PPS/FreePBX-Distro-10.13.66
(the dev box does not have the commercial System Admin module)
That went fine, but took an insane amount of time, not sure what server those updates came from but downloading at 30kb/s does not make for a happy me. Anyway, once those were done, the asterisk version was still 11.21.2, which caught me off guard.
11.22 has been out for a few weeks, I guess I just expected it to be in one of those updates. I didn't bother proceeding, I'll check the server when I get back to the office to see if the patch was removed or not (as the asterisk version didn't change, I expect the patch won't have been removed)
I've update the wiki with the newer version of mISDN and will update the asterisk and patch versions in due course, as it stands the instructions for 11.21-2 should still work fine.