Well, it didn't take long, but I have CHDK running accessing the Eye-Fi Card. For now, I just had it dump the firmware version string, but the sky's the limit! I should have it scanning and adding networks in no time.
As a commenter noted, CHDK requires that the lock switch be flipped before it can be run off an SD card. The Eye-Fi has no such switch. So, I took a chunk out of mine with an X-Acto Knife. Here are some pictures. I've also taken to sticking clumps of electrical tape over my new hole if I need to stick the card back in a non-Canon camera.
I also stuck some foil into the Eye-Fi reader to fake the lock tab because, with that chunk out, all the readers think the card is read-only. I could have done that with any reader, but my "official" Eye-Fi one was already taken apart. I wonder if I can override that in software. Anybody know offhand?
It took hacking my Eye-Fi Config program up quite a bit to keep it from using some of the facilities like abort() that are not available in CHDK. But, most of the code essentially stayed the same, and I integrated it pretty well. Now I just need to get some menus coded up to navigate the various options.
Now that I have bits of aluminum foil, electrical tape, razor blades ant toothpicks (for digging bits of tape out of SD card slots) I really feel like MacGyver.