The assumption is that once this onlay is on, you will epoxy coat and glass the entire thing over the veneer, correct?

You can use 15 min epoxy. Just put a thin coat on the veneer and once it gets fairly tacky, burnish it down in position. If anything squeezes out on the edges, don't use any solvents, try to clean it up with a single edge razor or utility knife blade. A solvent can leave a stain. I'm talking about 15 min epoxy in the matching tubes from the hardware store.