I was told once that air circulation usually goes from the back of the stage to the house to help the sound to carry -- once I started looking for it, that seemed to be the pattern, at least in the older houses. There's also usually an emergency venting above the grid, possibly those could be adapted, though it would hurt the sound and draw the audience air up across the stage.
Given the height of ceilings, I don't know if there's a ventilation solution for the house comparable to airplanes (like your suggestion) -- my guess is that if the theatres have to somehow operate amid contagion, spacing and tiers will be the order of the day. No more bleachers. Like the State Theatre or the Met, or even some of the smaller LES theatres that have one or two rows of seats per tier. Off-off could just set up chairs appropriately distanced. Perhaps one reason why the groundling-spaces were much cheaper than the tiers in Shakespeare's Globe.
~pcot |