the basic idea makes me nervous ..it always did.. even when using the rope. There's something about applying forces backwards (instead of the piston turning the crank, it's reversed) that bothers me .. but people do it and it works ok.
I mentioned an aluminum rod just because i've got some 1/2 inch stuff laying around. It's soft, drawn aluminum, and not near as hard as the piston alloy. But using anything less than a soft, wide cushioning pad like that afforded by a piece of rope, even a wooden dowel, seems a bit harsh.
For use as a piston stop to wrench on the crankshaft, i still think stopping it at mid stroke is best. It would be easier to break the tool or damage the rod bearings if the stop is near TDC for the reason i spoke of... forces against the tool will be highest near TDC. That small crank pin / connecting rod angle affords lots of leverage. Bearings and the piston itself seem more vulnerable than the rod.. but who knows..
As for a tool to detect TDC nothing beats a dial indicator, imo. The back and forth movement of the dial's needle lets you zero-in on true TDC. Changing something like head gasket thickness doesn't matter to the dial indicator.
this thing's a work in progress.. hashing it out on paper and in the brain saves a lot of effort. Adapting a old spark plug seems like a good basic idea to build on.