Didn't Wallace display a Sheetz rifle in one of the old JHAT articles (going by memory - I don't have them anymore) with original rammer that was spiraled?
I know I have seen a couple of rifles over the years with spiral rods that I am convinced were original. The caveats here are (1) they were all 19th century rifles, none that I would look upon as 18th century and (2) it clearly was not "burned" in, it always to me appeared to be aquafortis or comparable stain contrasting with unstained hickory.
One rifle was a Conrad Horn rifle that literally looked like it had been preserved in time since the day it was made and I frankly wonder if it had even been shot, ever. Another was a Samuel Baum in exceptional condition, and I know I've seen a few others here or there that I feel pretty sure - judging by the fittings on one or both ends - were original.
On the flip side, yes, how does one know *for certain* a rod is original?