Robert, the simplest registration system I know of is a couple of pins taped on plate glass, and then put another plate on top for exposure. It is very simple to make, go to any art store buy the two pins and a hole puch and you are in business. This can be made as elaborate or simple as you want it. For Gum, I am not an expert by any stretch, but I have done negative registration and I would guess just aligning the previous image with the negative over a light table is best, then just tape it. I hope Clay or Kerik see this, they do Gum over pt and they would be the best source for this kind of set up. You might want to e mail them, they are very kind and free with their advice and knowledge and so far they have been inavaluable to me for solving this kind of problems.
P.S. BTW Clay told me you have to shrink the paper before you do the first pass or the images will not registrate, so put the paper in hot water for at least 20 min and let dry before you do your first gum pass.