The screen dimensions of a TI-84 PCE are 320x240 — Identical to the lowest resolution setting on most computers, game consoles, webcams, etc. So, theoretically, one could write an assembly program to parse AV video, splice a Mini USB cable, and use your calc as an External Monitor! This would be strictly proof-of-concept of course, as there would be little actual reason to do this. Still though.

If you're thinking of just connecting a composite video source to a CE through a hacked up usb cable, there's no way it could work. Composite video is an analog signal, while the USB port is strictly digital. Even if you could read the analog data, I doubt the CE is fast enough to decode it on the fly. What might be slightly more possible would be to have some extra hardware between the CE and the video source, such as a USB capture card. You'd still have to write a driver for it, which would be no easy task.