***I wish I was..... (heavy sigh) I envy you.
***First of all, unless the temperature changes are rapid and extreme, if you leave the rig outside, you should be all right. I do this during the deer season here in Wisconsin and everything has been all right. Where you get into trouble is hunting all day in cold temps, then bringing the weapon into a warm building. That causes condensation to form. So I just remove the cap from the nipple and leave the gun outside. However, I only do this for a couple of days, not for an entire season. I can understand not wanting to discharge the gun in camp, plus, if you do, you will have to clean the run right away to prevent rusting.
Go down to your friendly neighborhood gun shop and get yourself a muzzleloader discharge device. This is a simple co2 cartridge triggering device with an adapter that pushes the charge and projectile out of the barrel with a blast of compressed co2. Point the gun at the ground, put on the adapter and bingo! The bullet and powder are on the ground, and you can re-use the bullet. End of problem......
I know one guy that uses a bicycle pump and a piece of plastic tubing, he shoves the tubing over the nipple and pumps the bicycle pump. The built up air pressure shoves the charge out of the barrel.
***You could, but I think my idea is easier.
***That's what I think.