I just tried flipping the Alinor Greenhouse, Summer upside down, which would make it much more versatile for, well, non-greenhouses.

Unfortunately, the hole you can walk through in the doorway goes considerably less high up than the visible doorway itself, so to use the thing upside down you have to settle with a fairly low ceiling height. Otherwise, you hit an invisible wall and can't get in.

If there's no in-game / engine reason for this, would it be possible for ZOS to fix this?

I'd beg, too, for doorways at both ends so you can line multiple greenhouses up, but I get that that's a more complex change.