Actually the #11 caps are more water proof then you might think. My Wolverine II made by Knight is still a #11 ignition and until this little rifle fails, I have no intention of changing to anything else. I load loose powder and it goes off every time. The Black Diamond XR I have is a 209 ignition. It also as you mentioned does have an open breech. You can get breech covers if you're worried about moisture. The 209 primers will give you a lot more fire into the breech and your ignition will be more secure. Otherwise I would not have a problem hunting with my Black Diamond XR with a #11 or a musket cap, although the musket cap does seem to fall off a lot.
Between shot swabbing is necessary depending on the projectile you shoot. I swab with a combination of isopropyl alcohol and car windshield washer fluid and it works great for me. If I shoot the 460 grain No Excuses Conicals I only have to swab about every 7th shot. If I shoot the 375 grain Buffalo Bullet SSB's I have shot five of them and then swabbed to make sure the accuracy would not suffer. About the only projectiles I have to swab every shot are the Barnes Expanders 250 grain which are excellent shooting, and to be fair, the 250 grain Shockwaves. I have loaded three of them without swabbing but it was ruff. A quick swab and they load a lot nicer.
This was done with my scoped Black Diamond XR at 100 yards off a shooting stick set. I did not swab at all during this. The 460 grain No Excuses shoot real good out of my rifle but that does not mean the same would be true of every other Black Diamond out there...