Yep. I found a place in the kext in /System/Library/Caches/com.apple.romextensions where the device and chip IDs for MXIC and PMC were next to each other. I changed one of them to my SST chip's ID, manually loaded the kext, and now the driver is working correctly. It's definitely checking the chip ID. It's mixed in with other data so I don't know if there are any other chip IDs present.

As far as I can tell, the chip has to be a Pm39LV040 or an MX29LV040 in order for the driver to load.

Edit: Found a SATA drive to hook up and it works on the SST chip flashed with ROMFILE.1S2! So the chip ID was the problem all along and I'm no longer afraid to mess with the original card. CC_333 should be able to flash ROMFILE.1S2 (or my newer firmware) with the DOS flasher and it should work.