I've spent the day looking at the TI tagged object file. The details are in the Editor/Assembler manual. It's a simple, but horrendously wordy format. There are smaller forms, namely the compressed object and memory image formats, but I have no details on those.
I was interested in getting an idea of how to make disk-based programs using the GNU tools, but it looks like I'll have a challenge ahead of me here. If nothing else, I need to get a copy of the E/A cartride, although Extended Basic should work too.
So back to GAS then...
Todo list for GAS changes:
PSEG and friends in addition to ".text" [DONE]
$ as current address [DONE]
Labels without ":"
Error if arguments are missing
Treat all after last argument as comment
Allow "*" comment symbol [DONE]