Before I start, here’s the TL;DR: Custom firmwares are not possible for Touch & Go without breaking open the device and possibly soldering stuff on it. Don’t try it - you’ll most probably ruin your device.
Now that we have that out of the way, let’s continue.
Out of curiosity, I wanted to find out what’s lurking inside the Touch & Go unit I have on my car. The device itself reveals very little information about its internals, but a firmware update image provided by the manufacturer was a great source for lots of interesting information.