Somebody suggested something similar to this before (though I can't find the post). It was the ability to apply text as a patch in the TEXTURES lump. I'm guessing you're looking for arbitrary text at runtime, though.
If this suggestion can be implemented, I think it would allow some very nice effects. Most of them could probably be done with actual textures but this would be a much neater, cleaner and easier way to do things.
This would also be cool for making computer terminal menus (RTC-3057 comes to mind) actually appear on the computer you're using. Would be sort-of like the Doom 3 computer terminals (minus the seamless interactivity of course).
Would be set up (on the modder's side) in a similar manner to a Camera Texture? Would it affect all instances of a particular texture, or just a particular one being drawn to? (As far as HUD sprites go, the latter would be more versatile in a multiplayer situation.)