Are there any gold plated dash Clocks and Timers out there, either
original or after-market?

Having remained on top of the dash timer market, I've never seen one.

Maybe presentation/celebrity items?

Nope

Would there be any demand from high end car owners, either singly
or as pairs/ triples/ quads, possibly with backplates mounted on
a polished hardwood 'toblerone'?

While no one can speak for everyone else who might potentially be interested, I would have no interest and, frankly, would be disturbed if someone altered dash timers by plating them. It would mean originals were ruined, which would be terrible thing to me.

Scott