OK, if I were to upgrade a non-disc folder to front disc, it does look like 20" disc forks are not common. But they are made for both the recumbent and mod trials markets, and are available in both 1" and 1.125":
http://hostelshoppe.com/cgi-bin/read...ory=1229623887
http://webcyclery.com/home.php?cat=326

If I were to do this route I would also have to make sure the A-C length and offset are appropriate for the bike in question, but I'll cross that bridge if/when I come to it.