You know I'm almost positive I recall that the TI-85 has a serial port which would allow access to the users program, but probably only with a proper program entered into the calculator might one be able to utilize the display in a arbitrary manner. So it might be possible but not sure of the benefit Vs effort required is worth it seeing how many inexpensive display options can be used with an Arduino. Still could be a cool project. Isn't the TI-85 driven by a version of the Z-80 chip?, least that is also my memory. However I am old and might have it all wrong.
Nah - that's right:http://en.wikipedia.org/wiki/TI-85
As far as directly controlling the display, I would say it isn't worth the time or trouble; it's a 128x64 display, thus a standard KS0108 128x64 display would fit the bill and be cheaper by far (in terms of time, if not dollars). With proper programming the serial port could be used as well(likely in Z-80 assembler, although the weird calculator "BASIC" TI used could do some interesting stuff - so maybe). There used to be a small robot kit you could buy (google "TI calculator robot" - it'll be the first few links for Norland Research; but I couldn't get the links to work) which controlled the robot via the serial port (I think for the TI-83?).