That goes in the right direction. I think the body of the first map can be moved to a resolve_kw function and that function can be reused in the second map. This way fancy cases like (;a=1, b, more...) are handled better.
I just noticed that AtBackslash.jl (my unregistered package) already supports it:
julia> using AtBackslash
julia> f(; kwargs...) = (; kwargs...)
f (generic function with 1 method)
julia> let x = 1
# input `()` is ignored
() |> @\ f(; x)
end
(x = 1,)
julia> (x = 1, y = 2) |> @\ f(; :x, a = :y) # more real-world usecase
(x = 1, a = 2)
(This is only relevant for anonymous functions, though.)
Julia’s AST is very well-designed so that you can accidentally implement features 
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This is implemented in the language itself, since v1.5:
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