Here's a halfway house suggestion for barometric correction without using a second sensor.
There are plenty of 12v solenoids available out there from vacuum piping used to control emissions setups on cars. Why not incorporate a switchable barometric correction, enabled by a momentary (push to enable) switch, which could divert the MAP sensor to the engine bay rather than to the inlet manifold while the button is pressed. If available to be used only at idle speed (and perhaps because MS had noticed that idle speed/settings had changed sufficiently), the engine could continue to let MS use the curent MAP setting to idle, whilst it samples a new ambient barometric pressure. Once the button is released, MS continues as per normal, sampling manifold vacuum, but has a new reference point rather than that that it has had from startup if it needs it - i.e. if barometric correction is needed , it'll implement it.
In theory, it could be implemented any time the revs/engine load are constant.
There are no hills where I live, but it was just an idea.