Something to consider about your typical teflon tape is that it's about two ten thousandths of an inch thick.
That's .0002"
It'll take quite a bit of that stuff to make a difference, but for what it's worth, I'd guess it's not likely to flow or burn or allow lead deposits to occur on the rifling when used in blackpowder bore rifles. That may be the benefit right there.
Getting it to stay put while seating the bullets might be another task in itself.
I haven't tried it, although I have used plumber's teflon tape as a sub thou thick shim in the machine shop numerous times.
The teflon might make the bullet 'slicker' too, which might end up calling for some kind of load adjustment, might be just the trick to getting the barrel's groups to converge sooner than later while working up powder charges too.
Things to ponder...

I'm interested to hear the notes from your friends, 500

For what it's worth, I'm still working on a load for the new bore rifle. Shot the Mahillon today with stellar results. I'll be getting it out again later in the week to work it at a hundred yards.
I'll likely have the other 16b sxs out there working on it too.

--Tinker