Originally Posted by danix
Yes, unfortunately passive dongles only support single link. To do what you want with active dongles, I'm pretty sure the dongle would have to allow link training even if a display device isn't connected. I'm not sure if there are any existing active dongle designs that do this.
If I was running a newer X server, would XRandR be able to dynamically add the display back in? That way I could have a udev trigger that just seamlessly rearranged the display.