I don't think you'd need to use an A-series SOC, considering the power usage of even a M4 is basically a rounding error - and they've already got M-series stuff running passively jammed into a tiny case anyways.
I'd be on something like that immediately, but I somehow doubt Apple will ever make a 12" Macbook ever again, given that the majority of people seem to like the 13" airs just fine.
Love to be wrong, though.