My solution is to use Arca Swiss type plates on body and lenses with tripod collars. I use Blackrapid straps, with a Kirk 1" clamp on the end, so I can easily connect it to body or lens, or remove it for tripod/monopod use.Is this the Kirk clamp you use ?
Having not used Arca Swiss type plates before they do not look suitable for holding the camera upside down on a strap. The sliding that you have on the plate I thought would mean if not really tightened would work its way off the camera?
That's the one. The clamp has good intrinsic resistance - it doesn't open up on it's own or with a casual brush against something, you've got to grab the knob and apply some force to loosen it. I attach it with the knob under the lens with a body plate, and away from my body with a lens plate, so nothing will brush it anyway. In 1.5 years of use, I've never had the clamp loosen up even slightly on it's own.