I can't help with your cup question, YourDadsMistress, I haven't been too adventurous with the uses of it, but as for the paint pallet thing, have you tried a circle cutter? Fiskar's has an adjustable one that I purchased because I have button makers, but it works for this as well. You can make circles until you get the size you want (so that it sits where you want in the resin) and tighten the arm to "lock" it in place. I even glued the arm on one of my circle cutters (I have 2 or 3) so that it won't change size when I go to cut out that particular size for buttons. You might be able to find a circle punch that works, too, but I think the circle cutter is a good investment because you can adjust it to fit whichever size you'd like.
What an amazing idea!! I love it! I've got the same issue with mirrors--I've had the same dresser since I was a little girl and always had to cover it up at night, and eventually my dad took the mirror off. I still do not have a single mirror in my room. This is just a great idea, though, and it looks beautiful!