Could you add a tinker modifier for the sledge hammer in order to make the hammer grind compressed blocks?

This is something I've considered in the past. I will say half the point of using expanders on the hammer is to break more blocks, though I could see a modifier like you mentioned making sense. For it to make sense in the context of Tinkers Construct, it would probably replace the normal hammer drops in exchange for the compressed ones

It looks like the latest version of Ex Nihilo added their own version of the Tinkers sledge hammer, so I doubt I will be keeping that feature for 1.14. as a result, since I don't plan to do any more major features for 1.12 I doubt I will add this. I suggest you look into expanders which provide a similar effect (3x3 area), or I would be open to a PR that adds this before I stop making releases (still have a bit before Tinkers Construct is ready to port)