Neat experiment
I do all my macro work (lots of it too) using extension rings, usually together with my 70-200 F4/L. My partner does the same with her 28-135. DOF is a normal problem with macro from our experiences & reading, whatever the lens combination. Every single shot is a careful balance between desired DOF, required exposure time and brightness.
I'm lucky that with digital I can just bump the ISO up to 800 if I need to, she gets stuck with the 100 speed Velvia she has in the camera
I haven't tried reversing a lens, but have read about it, it would be interesting to try! What I would be more interested in trying is a 100mm dedicated macro lens. I have always wondered what the difference in DOF and image quality is between that and using extension rings.
Roger.