This is a silly and probably much too laggy of faking it, but you can run scripts from the main menu, correct? Just place a separate piece of scenery for each individual frame of the animation, then, using scripts, delete them all at the UI startup. Once they are supposed to begin "animating", you object_create_anew one individual frame, then delete it and object_create_anew the NEXT, and so on.

It would be laggy, probably, and I am more than likely overestimating Halo's engine, but it WOULD be a way to fake animations in a UI, short ones at least.