Below, cutting the teeth away from the rubber jawset as neatly and carefully as possible, and then cutting holes through the entire thickness of the piece. Then, inserting it back into the original mold. The idea is to pour white resin into the mold for teeth, remove the blue part, and then pour pink resin for the gums.
Below, the final product. Not too bad. (Tangent alert- the stuff that looks like pink slime is in fact pink slime. When resin is overloaded with colorant it will not cure properly. I was using Smooth-Cast 65D here, a white resin, and I had to put in too much colorant in order to get a color other than very pale pink. The saints at the Smooth On tech support department recommended I switch to a transparent resin, Color Match 325, which I did.)
After I made a bunch more of these I decided this method may not be everything I hoped it would be. Sometimes pink resin would leak down over the white teeth and while I could carefully scrape it off before the resin set completely, this was something I'd rather not spend my time doing. Also, I would get thin spots or bubbles between the teeth and the gums at times, which would make for weak castings (I could crush in these spots with my fingers) which was definitely something I did not want. Time to try another method. Onwards!