Well, I've done a little digging, and it seems the Sat Nav system runs on a Motorola 68000 chip, and the OS itself is stored on the OS CD, and not on the device itself! If you check out mmi_sw_load in a text/hex editor you'll see a bunch of bootstrap error messages in plain text a little ways down.

The OS itself is Microware OS-9 - not to be confused with Mac OS 9. Thanks to the power of 'teh internetz' I have a manual for both the chip and the OS, and am currently downloading an IDE and disassembler.

With any luck, I should be able to figure out a way to customize the menu without screwing the actual navigation up...