yeah the dremel may work Barge but you risk tearing the thin veneer, you may have success with this method. Before you glue the veneer I'd trace the body shape and with a pencil shade a piece of paper and try and mark out the rough shape of the cavity, so you have an idea where the edge of the cavity will be, and include the tremelo cavity. Then sticky tape the cavity shape paper to the other piece of paper to the body shape in rough position so you have an idea.
Once you get a good size hole in the veneer it will be much clearer where the edges are.
I'm guessing the veneer wouldn't be cheap so you don't want to stuff it up. Also make sure you get even pressure when gluing the veneer. My uke veneer got some glue lumps where there wasn't enough pressure on it.
So we're up to page 4 bro, keep er rolling ! haha