I've got an absurd amount of time to devote to reverse engineering these calculators. But I've been having a great deal of difficulty discovering how the knowledge about IO ports was gained in the first place. As my eventual project is to completely reverse engineer the whole OS knowing the ins and outs of every port used is a huge requirement. How was the detailed usage documentation created? If anyone knows I'd love their help!
Brandon W would pretty much be the person to ask. His website is here: http://brandonw.net/ . There is also a good amount of info here: http://wikiti.brandonw.net/index.php?title=WikiTI_Home . He also tends to idle in #Omnimaga if you want to catch him on IRC. I don't think that he's registered here or if he frequents this IRC channel.
BrandonW is actually idle in #CodeWalrus but he indeed doesn't have a forum account. I don't know if he is very active, though. Perhaps he checks #TI more often.
Official TI docs, OS disassrmblies and thorough testing by hand were most likely used for that.