Never tried it, but if pressed I'd have a try with some PCB acid-resist etching. If you've got access to a laser printer it's easy to transfer the toner from a printed design to the etch resist film, this is then transferred to the metal to be etched and the PCB is soaked in dish of Ferric-chloride solution. Perfectly safe to carry out (ask a grown up for help!) in your own kitchen. Things have moved on a long way since I used to do this with Letraset and a magic marker.
Not quite the same but I always get plates made for any race bike engines I build. I use Lexicraft http://www.lexicraft.co.uk/ I don't know if they'll do one-offs but I tend to order 100 plates at a time with sequential serial numbers.
