I remembered reading this thread a while ago when I was having the same dilemma, and just thought I'd resurrect it to share my findings after using a sling strap clipped onto the QR plate for a while now.
I've been doing it for a while, using a 5DII and usually a 24-70 or 135mm (nothing bigger/heavier), and a sun-sniper strap, (probably comparable to the Black Rapid RS-7 - it's got a curved type shoulder pad). Whenever I'm using it I've always got one hand on the camera just in case, but I'd do that with any strap really!
In the few months that I've been using this setup, at no point have I seen anything to suggest that the redsnapper QR plate isn't strong enough for my particular setup. There's no sign of fatigue or anything, but I suppose there might be things happening inside that aren't obvious.
What IS obvious though, is the wear to the strap's connector:
Now, most of this was done in just
one weekend. Beforehand, there was a little bit of wear, but a weekend of trekking around a welsh mountain did this. I think another weekend and I'd be seriously starting to worry about it's ability to support any weight.
On the bright side, the RS QR plate is made from some seriously tough stuff. There's not even a hint of wear to the loop on the plate, it's still got those pretty sharp square edges that just ate through this connector. A bit of the black coating has been rubbed off, but that's all.
So for now, the sling strap has been resigned to a drawer. I'm searching for a new connector thingy, then I'll adopt this very cunning solution, which still allows the same flexibility:
I dont know about the redsnapper plates but I use a Arca style QR clamp attached to my BR strap - I can then attach/detach from the lens plate.
Attached is a pic of the set-up - I use this with a 7D + 300/2.8 + converter (almost 8 lbs)
Hope this helps to anyone still interested
Chris