I don't know how anyone will truly ever know, unless it's possible to just ask somebody that did work on the project.
I know Dylan Cuthbert talked quite a bit on the software side.
Maybe one of the hardware or systems programmers will be willing to talk now (sometime?)
here's one of them (and you can watch his cat videos)
http://www.youtube.com/user/CarlNGrahamMaybe ask if he would be willing to answer a list of questions to be sent privately, after you explain what you're trying to do and why it's important? I guess if it seems like work they can get a little bit cranky so you have to respect that. But honestly engineers are pretty approachable in comparison to say, TV producers.
I have read long posts from the TI-99/4a TI guys who were eventually willing to talk, and I'm sure their comments have saved thousands of man hours of struggling.