I am not aware of such hook (it may be my ignorance of course). That would allow one to write a custom preprocessor which operates on the string level - or you can call it a compiler if you wish - which will take a string of whatever syntax you design and generate Mathematica code from it. It would be easiest if there would be some hook like $PreRead, which would allow the user to intercept the input string and process it into another string before it is fed to the parser. Additionally, there will likely be difficulties with using it in packages, as was mentioned in the other reply already.

As for the tools: I have very little experience with Notation package, but, whether or not one uses it or writes some custom box-manipulation preprocessor, my feeling is that the whole fact that the input expression must be parsed into boxes by Mathematica parser severely limits a number of things that can be done. Regarding the stated goal - I support the idea very much, being able to reduce verbosity is great (for personal needs of a solo developer, at least). First, a disclaimer - I don't suggest any of the methods described below as good practices (perhaps generally they are not), they are just some possibilities which seem to address your specific question. Not a really constructive answer, just a couple of thoughts.

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