inside Experience there is an option to customize a display for three types of monitors, computer screen, tablet and mobile. In tablet and mobile configurations there is only a Portrait configuration and it is not possible to adjust a display for landscape viewing. Is there a way to make such an adjustment?
I know this really doesn't resolve reactive portrait or landscape design modes, however, I've found that if you structure your tablet view to be landscape and portrait "flexible", mobile landscape will use tablet landscape and look ok.
Again, realise it's far from a solution but may help you muckle through until a solution is incorporated into ExpB