Let's stick with your original idea for a while. The only sites I can find which sell the 'bake in the oven' version of this product are overseas. There's a UK alternative, however, which almost certainly produces a far superior result to the DIY method.
The way it works is that the firm send you your chosen plate, together with a special pencil. (You get to choose both the style of the plate and the central motif). After you've had the plate signed, you return it to the firm (using the special packaging provided). An artist then improves the legibility of all the signatures by carefully painting over each of them in black paint. The plate is then professionally glazed in their kiln before being returned to you.
It's certainly not cheap but, given the choice of spending the money on international shipping for the basic product or spending it on genuine craftmanship for a superior product, I know which I'd go for:
http://www.allfiredupsigningplates.co.uk/
(Work your way through the links down the left-hand side).
Hoping this helps,
Chris