Although I'm not a buckskinner, I've personally witnessed a fire started with a flintlock, but it wasn't done in quite this way. Instead of char cloth, some other tinder was laid across the charged pan and caught sparks when the powder was fired.

If you've got char cloth and a flint lock, you really don't need the powder, since the char cloth will catch a spark really well. The sparks off the flint should be all that's needed.

YMMV