I think you have to consider them just a novelty. I'm sure those offered are genuine, but if they ever traded for a lot, fakes would take over. How could you tell a shield nickel planchet from a Jefferson nickel planchet ? I've never seen a planchet offered for a gold coin or even a silver dollar.
Indeed it is the easiest to fake! Fixing the alloy content too will not be terribly difficult as you can melt down previous old coinage and reuse it...
The shield nickel planchet is smaller in diameter and thicker. And sure you can fake a planchet. All you need is a furnace for meting the metal and casting the ingots, a rolling mill for rolling out the strip, a blanking press for punching out the blanks, and an upsetting mill for converting the blanks into planchets. For only ten to fifteen thousand dollars you can be in the fake planchet business. (I guessing here, I think there is a good chance my estimate is low.) I have seen gold coin planchets and silver dollar planchets, both silver and CuNi clad.
