I had a finger snap off of an ultracal hand mold. Can I just use something like gorilla glue or epoxy to to secure it to the hand?
CA glue would work, as would epoxy. You may want to drill the pieces and insert a rigid wire for some extra support.
I've had good results repairing ultracal with Rustoleum Concrete Patch & Repair, an epoxy designed for cements. Definitely do drill out the halves and reinforce as Chris said. For that I like to use a piece of threaded rod, as the threads give you some extra texture to lock into. Ultimately the fragility and environmental susceptibility
of stone molds is the reason I've switched almost entirely to composite epoxy molds. It's removed so much headache from my life. Basically doing variations on what the Immortal Masks dudes taught in their silicone mask course for everything now. Helluva course! Thank you Winston School!