A thought just occurred. I'm dealing with sense pins, right? How about just putting a paper clip, trimmed just right, into those pins where the mac senses them? I'd bet they have to be grounded, but take a look at this blog, and you'll see where I got the idea from.

http://www.hackaday.com/2008/04/26/ghost-external-vga-display-hack/

Anybody got some specs on the sense pins of a IIsi?