The problem with doing this is that someone who later reads the code would probably think that the expression contains an actual function call, rather than being an identifier. Or they could confuse assignment with function definition (since e.g. x+y = ... redefines the + operator.)
Even if there should exist a technical solution to this, I’m not sure if there is a good solution.
BTW, there’s a related (and simultaneous) discussion here: Add unicode symbol ⟋