Just come accross this thread, and thought I'd add my (very limited) experience with this.
I recently cleaned up an old Ferguson radio which needed repairs to the tuning scale. I scaned the old one, cleaned it up in paint shop pro, then printed it onto waterslip transfer paper (available from
www.craftycomputerpaper.co.uk). Now for the clever bit: I printed a second version on plain paper, and used a craft knife to cut out those bits which were meant to remain clear. These were used as a "mask" on the back ofthe radio glass, which was then sprayed with white paint. This had the effect of "beefing up" the main colours (which were a little too tranparent once the transfer had been seperated from the backing paper), but would also leave any un-masked "clear" areas showing white.
Hope this makes sense!