You might try printing small labels with a laser printer then attaching
them by pasting them on using the last layer of finish (if you put the
label on with first coat, you risk sanding through it). Since the toner is
actually tiny beads of plastic that are fused together, it won't be
affected by solvents (well, none that won't attack the wood too..). You
can scan in signatures and logos and encorporate them into a design using
Word or Open Office. If you can find the right kind of paper for the
finish, the paper becomes almost transparent.