Hey guys, sorry for posting on a dead thread, but I just wrote a TOTP implementation for my TI-89, and I found this via Google. I wanted to let you know that it works!
Unfortunately, my code takes just over one minute to generate an OTP. This is using the TI-Basic language, which is really bad performance-wise. Plus, the bit shift and bit rotate functions keep generating warning messages. You can write programs for the TI-89 in C, so going that route would probably give you an answer in a more usable timeframe.
If there's still any interest in this subject, let me know. I could grab my old cable and upload the files for anyone else curious. Heck, I might take a stab at the C rewrite as my next project.
Edit: Here it is! https://www.github.com/divergentdave/TOTP-89