Well i suppose my method works pretty well. My bags are a year old and have spent lots of time in the sun as well as rain, and they both look clean and black as can be.
I have a leather cleaning brush (anything with soft bristles) that I use with some mild soap to scrub off dirt and bugs, and then after rinsing and drying really well, I waterproof them. 303 Fabric Guard is similar to what you guys are thinking about, it's designed for boat covers and seats, but works equally well on all outdoor fabrics. It also has UV protectants, which we all know are vitally necessary with how blue these bags get with age. I flip each of the bags open and lay them flat, and fully wet all the fabric with the 303 fabric guard. A couple days to dry/cure and they're ready to go. I planned on only cleaning them when necessary, and waterproofing them once a year. So far so good.
Yep 303 all the way for me, I bought 5ltrs for my sports rib Hyperlon tubes and seats, for it's superior UV protection properties. works well on the bags