Based on the prequel documentaries, Lucas seems to delegate a lot of work and then gives his input sitting in front of a monitor. He also might have been too busy with Episode III to be shooting new bits for THX.

This is about the only making of article I've been able to find.

http://www.awn.com/articles/people/back-future-ithx-1138i/page/1%2C1

I'd also like to think he wouldn't personally make a silly mistake like the Robert Duvall body double having his hands in the wrong positions on the monitor added to the "report violation" scene! ;)