I have battery grips on both bodies, and the grip provides the lower attachment point for the strap.
Yes, that's the Kirk clamp I use (I chose it over the small RRS clamp because the latter has little nubs on the bottom where the FastenR-3 goes). I attached the FastenR-3 to the Kirk clamp with a bit of Loctite Blue on the threads. I do have a spare FastenR-3, but haven't had any need to use it - the Kirk clamp makes it so easy to change where I attach the BR strap.
It is interesting you mention the RRS have the nubs that would interfere with the Fasten-R3 - check out the link, the forth picture over has exactly that combo:
The nubs are likely to mate with a BH-25 or the like, although that forth image implies that the nubs have been removed.