A first way to add an external 240x128 screen to your TI-89 Titanium is to build an USB adapter like the one from TI:
See here for additional info:http://tiplanet.org/forum/gallery/image_page.php?image_id=1103
Note that it's mainly build around a FlashROM whose content is unknown to us - which might not be easy.
Another way is to start from the internal J3 connector:
See here for additional info:http://tiplanet.org/forum/viewtopic.php?f=42&t=9658