I know you are looking for a specific answer to a question, but if you look at the Think Tank Camera holsters (Digital Holster 40 v.2) it has many features - including a rain cover.
Digital Holster 40 V2.0
I use them exclusively and going to the "Bat Belt" system was one of the best decisions I ever made when it came to rapid access to my camera and gear.
The zippers, construction and little features of these holsters are truly wonderful.
They are in a class by themselves when compared to other holsters that are out there.
I wish I could explain the feeling of the zippers, how the fold out design, the pockets that are elastic and not, the lens cap pocket, the D rings...these puppies have so many features!
Far superior to a back-pack based solution.