Thanks for the help!
I went back to basics, and used the Applescript tool to reformat the card and intall the CHDK. The newest build boots up fine in "autoboot" mode with a locked SD card, so I'm all good! It still doesn't show up in the Canon menu if I want to maunally load it, but I can work around that.
I've been using CHDK with an Eye-Fi card, so I may just take the plunge and file down the plastic where the write-protect notch should be.
-John