I read once where you can fill a small syringe with grease and seal up the pan once it's primed; never tried it though. I carry a silicon treated cloth sealed in a bag in my pack. When the weather calls for it, I drape it over the lock. I also sweep the old powder out and re-prime every hour or so. With a flintlock, there are times it's smarter to call it quits.
If you're having trouble hunting for deer,
don't wish things were easier;
wish you were smarter.
My grandfather sweared by using petrolium jelly around the lock. Never tried it myself.
I will say this though. A few years back on the last day of the season it was really warm and raining hard. My neighber took his flintlock, and I took my shotgun for grouse. He said he just wasn't going to put powder in the pan until he was ready to shoot. I told him he was nuts, and would have to pull the pall. Around noon we were both soaked to the bone, and decided to call it a day. Walking out of the woods he stopped, and I kept walking. I just about jumped out of my skin when I heard BOOOOM.