Does anybody know of software-defined ways to switch this behavior? I have the opposite problem sometimes -- I have an external display shared between two devices (of varying OSes), and sometimes I want it to remain a logical display on each device no matter which input is selected on the display. But other times I want the display to be removed when the device is not the active input.

I'll bet that SwitchResX will do it (for the Mac). It's a pretty heavy-duty utility (look at the link in the root).