After thinking a bit on it, I will not miss it anytime if it will not be implemented but, at the same time, it would not be so harmful in the case it will.
For the later, I’m in favor of the original spec:
h = {x:, y:}
p(x:, y:)
or the underscore one
h = {x: _, y: _}
p(x: _, y: _)
Both of those aren’t strange (for me), and are clear and concise.