Although various nichol brass alloys have been know for centuries the process of manufacturing it was not really perfected until the 1820s in Germany.
Unfortunately that doesn't answer your question. The invention of a manufacturing process and the commercial availability of the product for gun mounts in an American longrifle are not the same thing! I don't collect or really study longrifles from the 1830-50 period but that is when I'd expect to see German silver mounts showing up OCCASIONALLY.
So ask yourself if you are building a flint rifle from that late and if all of the features, not just the mounts and lock, work together for that period.
The question about whether your friends will chastise you or not comes down to who your friends are.
If you follow this web site you will have noticed that there are a number of folks who regularly say something to the effect of "It is your rifle so do what you want as long as it makes you happy."
Gary