I'm thinking something along the lines of
"You're tough enough for this. Expect the same from your camera."
or
"Ever missed a shot because your camera was too weak to come?"
or something similar... I like your mockup John, but I think the key is to position it broader than just rain. Nick's shots certainly add one dimension which I think is quite important: cold weather. Pentax should be the TOUGH camera that can handle a bit of abuse and happily keep shooting through it.
I think the direction is great, and hopefully they're listening in... Wish I wasn't swamped with my regular clients or I could spend a little bit more time at it, maybe even involve a creative team at the agency...
Thomas