Here is what i think why they hold back on this support of multiple window/views
If in the camera.cfg it is possible to change the eyepoint view its in initialpbh and initialxyz section
But unfortunatly it is related to cockpit and not to the horizon although it move the frustrum a tiny bit, but i think it is not enough if you angle 22.5% of angle, in fs2004 and i think in fsx aswell it was the other way around it relayed the frustrum at the horizon and not in the cockpit.
I think the danger in this lies, if they would make it like that a lot of people would experience some more problems on how the sim will behave.
A solution could be if they could rewrite this outside of the sim and sell it as somesort of patch to upgrade it to a pro-like version.