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.
Lefty