Were you to use a slide copier and separate onto B&W film, it seems to me
that you would only have to worry about registration at the last step,
that is, when you make the final full-size negatives which will be used to
contact print your gum images. These separtion negatives could be
placed in register, and marked or punched for future use.
I am sure this would work fine because some years back I made some
three-color gum prints this way, and registration was not a problem.
Other things were, but that is a different story.
Sandy KIng