It looks to me like the only practical option to prove the concept is to Grendelize an RPD light machine gun, then put it up against a vz59 or PKM.
That would be rather expensive, though, so I'd suggest a more modest first step: Buy an RPD belt and see how well (or poorly) 6.5 Grendel rounds fit in it.
IMO, a second (or parallel) step would also seem prudent: Do comparison testing of 6.5 FMJ vs 7.62 FMJ. Comparing barrier penetration and other characteristics would confirm or refute the idea that a 6.5 LMG can deliver the desired performance.
If those steps produce acceptable results, then consideration can be given to acquiring and converting an RPD. (Perhaps a Grendel Guild project? Maybe DSArms could be convinced to help?)
Comments? Other proposals?