Urac will release if heated, but it isn't as cooperative as epoxy. I get success about half the time, the rest I over-dry, scorch, or whatever.
Glass on only the back will move the neutral plane forward (putting more of the limbs thickness under compression, hence putting the belly under MUCH greater compression) and that might be too much for hickory.\\
I'd look for either a way to improve your hickory backing (better selection or sawing or whatever), or try a natural material like the others have said. Rawhide, cotton or linen fabric, , etc would do great. Fabric backings are so easy....and can look great!