To make the Grigri function properly you need to redirect the tail upward. It would be a huge waste of energy to do it by hand, could be done but I think you'd get tired off it pretty quick. You could substitute a biner for the pulley but having it run nice and easy over a pulley adds enough efficiency to make it worthwhile. Here's another way of looking at it. Climbers who use a slack tender on their DdRT system can install a pulley or just redirect the tail though the biner that holds the split tail. I've tried it and I always go back to using a micropulley. I think it's the same for RADS pulley, you can go with out it but it's much better with it.
Jim Dunlap on the Buzzboard has his rigged with a pulley with one cheek removed, that way it's quick to take the rope off when you need to. The downside is that the rope can jump off the pulley at the wrong time.
-moss