Clamps approved for direct burial are also approved for concrete encasement (see the UL White Book "KDER"). i don't see anything in the UL listing that would prohibit the use of an acorn clamp to rebar if the clamp was correctly sized.
IMO, bending the wire over is going to have to be a judgement call on your part, though I don't see why this would need to be done.
A question for you. If someone hammered in a piece of 5/8" rebar as a grounding electrode, would you accept this per 250.52(5)(b)? If you did accept this, would you require a special "rebar" clamp or would an acorn do the job?
Personally, unless someone can come up with something really compelling, I'd continue to accept the acorn clamp that is listed & marked for direct burial. I think your years of experiance in this area are correct. I'm also sure that if I'm wrong, someone will kindly point out the error of my ways.