This can't be done in CHDK. Loading CHDK is done by Canon code.
Once CHDK is loaded, the Canon OS is trashed, so even if CHDK could detect which camera it was on, it would have no way to load a different file. It might be theoretically possible to hijack some romstarter code to do this, as it doesn't vary much between cameras.
For larrycafe's specific case, it's even worse: The cameras have different diskboot encodings. When the canon firmware encounters a diskboot with the wrong encoding, it just loads it anyway and decodes it with the wrong key. Then it tries to execute the resulting garbage... in this situation, there is no possible solution short of modifying the camera ROM.
You can have two different builds on a card if you are willing to load one manually. Just copy the PS.FI2 from the one you want to load with "firm update" manual loading, and the DISKBOOT.BIN for the one you want to autoload. You'll have to unlock the card before putting it in the one you want to load manually.
You could theoretically add a third (manually loaded) version to the mix using the canon basic loader.
