I thought about those problems myself, but are they not solved by doing all assigning of cuntion results within some "HDL namespace"? We define that namespace, and do all operations to variables using that ns:

ns.y = function(ns.x)

The question of what function(ns.x) returns goes away-- the ns.x object knows what to when function is called on it, and ns.y knows what to do with the return value of any function that makes semantic sense to be assigned to it.

Other than the problem of "I have to remember to assign function results into an HDL namespace, and not a regular variable" (which I agree is not fantastic, but it certainly isn't a slam-dunk justification to add new operators), where is the problem?