I have an RS-4 for almost two months now and used it alot during a three week-trip to the US. I carried it with a backpack on dayhikes in Yosemite and the FastenR-3 never worked itself loose.
How have you managed to get both a backpack and a black rapid strap on at the same time?
I actually have done the same thing. I put the BlackRapid on first, then my backpack. The straps to overlap on the left side but there is no conflict on the right. It was just like using a BR strap normally.
Blackrapid first, then your backpack. you can even use the waist and breast straps of your backpack. simply close them under the Blackrapid and it works perfectly.
Keep in mind to take things like batteries and memory cards out of the small pocket. It's very annoying when your backpack's left shoulder strap has to be adjusted every ten minutes