81.5: . . . but only after you install the first operating system update in years. Also, it will usually spoil in less than an hour.

41h: One-quarter to one-half of all memory should be permanently reserved for OS usage. Furthermore, one-quarter to one-half of that reserved memory should be unused, but never exist in contiguous blocks longer than 768 bytes.

Does PTT force a garbage collect?

Languages that are recursion heavy tend to implement something known as tail call elimination and other techniques that eliminate the excess memory consumption of recursion.

There are a lot of tokens. Just press 2nd+0 to bring up the Catalog to get a list of most of them. If you can get a script of some kind to extract tokens from ti83plus.inc, it'd go a long way toward getting the program done.

The (crystal) timers don't need interrupts to work. You can check them passively. I assume the issue here is being able to force the link lines high/low at the right time when you're using them passively. Quick research on Wikipedia suggests that if you get the pitch constant to within 25 cents, the variation in pitch will be acceptable, perhaps unnoticeable. To provide, say, 8000 Hz audio, you'd have to get your code to check and output 8000 times a second, with a sample-to-sample time variation of no more than 1.1%. . . . Actually, these numbers are starting to make this sound pretty hard. With interrupts, the timing is easy, but without them, you'd need even way more checks than for doing the grayscale.

The TI variable format is pretty easy to read and write; the TI Link Guide found on ticalc.org has documentation on the format. The challenge will be tokenize/detokenize code, and generating the look-up table for the token's text.