Possible fantasy methods for a sap-eating mammalian creature:
1. A sharp tounge that acts like a stylus, drawing sap from the tree.
2. A rasp-like tounge for licking up sap spilled by wood-gouging teeth.
3. Nothing special about the mouth, but a long finger used to collect sap from the wood.
I'd not worry about six armed mammals and the splitting of the radius and ulna, although a creature that has four hands but only two upper arms leads towards an interesting description.
Oh, and another creature that sticks egg sacs to trees is the spider, with strands of webbing.
Demiurge out.