Rob, I am curious about how you are going to retain the coin in the cheek rest, Mostly with inlays such as this are held with small nails of the same material, and flushed with the surface.
I think if I were to do that, I would be tempted to use glue, but in very cold weather, most glues would be prone to pop loose with any bump or jar to the stock.
I collect 19th century coins, and have a lot of Morgan dollars. Heck, I even have a $5 gold coin from the 1840's, which would be period for the Hawken FS I'm building. It is about the diameter of a nickel, so wouldn't be difficult to inlay, but to me the coin is worth more as a collector's item. It is soft enough to make a nice sized sheet stock - if you don't mind halving the value of it.