Just a followup - I found a source of cheap 631-type motherboards. They're listed as Performa 580 motherboards, but they have two sockets and should take one 128 meg and one 64 meg SIMM.

http://www.adoptamac.com/product_info.php?cPath=31&products_id=53

Note that the site is listed as one which contains malicious content, but they have Google Checkout, so even if the server isn't secure, you can still get one for $7.49.

I'm putting this motherboard in my Quadra 630, so anyone who has a shell on this machine will notice a little more memory soon.