I've never hunted in a downpour with my flintlocks but I've hunted in blizzards with nothing more than a leather, waterproof cow's knee protecting my flintlock. I can remember standing on watch with my rifle lying horizontal on two limbs of a fir tree with 6 inches of snow from muzzle to butt. After hunting, it went off without a hitch.
Back in the late seventies, I bought a little bottle of a Teflon material called "Fuzz's Dry Pan." I still have and use the small bottle of fine, flaky material. Directions said you were to mix 1 part dry pan to 6 parts powder. I never mixed it that strong. I'd measure out some 4ffff powder in a baby food jar and mix in some Fuzz's Dry Pan. Shake vigorously and you'd have enough priming powder for several years. I'm still using the same mixing jar as well.
That material was manufactured by Fetterman & Sons, RD4 Box 381, Newville, PA. I doubt you can find it today. However, another product that I think might be the same thing...possible the same formula, showed up on the market several years ago. It's called "Fletch Dry" by RMC...Rightnour MFG Company. Their address is Main Street, Mingoville, PA 16856.
Last year while up at the Sportsmen's Show in Harrisburg, I stopped at the RMC booth and talked to them. They have a new product called "Dry Pan" that they assured was the same formula as their Fletch Dry. I forget how much it was a bottle but a little bit of this mixed with your priming powder will surely extend your days afield in inclimate weather.