how can I export a video that will loop indefinitely when played from a usb drive?
As far as I know, PPro does not have that capability
If you were creating a DVD in Encore, you could put an "end action" on the video timeline to itself... so at the end of the DVD video it would go back to the beginning
I am not aware of any way to do that for a simple file
You can create a Timeline that repeats the content as many times
as is possible to export to a file of your preferred playback format
that will still fit on the USB drive.
If you have a big enough drive, this single file, repeating playback
might run for enough time to fit your need.
What other inputs does the TV have? Perhaps you could hook an iPod up to it and set it to play a "playlist" (containing just one file) over and over again. We did this here for an art installation a couple of years ago. It wasn't flawless (iPod needed restarting sometimes) but it worked well enough for two weeks straight