I get it now --- you're basically looking for a way to rotate about a "hinge" that will not likely be axis-aligned.
Agreed this would be a nice feature --- barring someone writing a plugin, the only way I can think to do this would be to rotate the entire door until its hinges are axis-aligned, then rotate it about the center of where the hinge would be, then essentially "undo" the first rotation to get it back where it started. Which it sounds like you'd like to avoid
