The maturity would have nothing to do with it, as far as I can see. I've grafted over 400,000 plants either with my own hands, or through people under me in my nursery manager days, and although I've never done a Nep, those long European cucumbers get so large because they graft them on to squash seeds when just a few days old.
Now I will make a guess, just for fun. You would use the whip and toung method. This is a long diagonal slice, then a thin slice just under the first. Then the same on the other plant and fit together. I'll post a photo when it gets lighter unless someone who has actually done it chimes in.
Very nice thought, though, and I cannot see why you couldn't graft a slow growing Nep onto a fast growing Nep and get something that would grow very fast indeed.