I've just recieved a DOD stereo chorus, and someone has replaced the 9v in with a headphone jack. Unfortunately, the PCB is mounted to the jacks and i can't take it apart at work, but potentially, could this be a stereo jack with inputs jumpered from each output?
It could probably be done quite easily. All that is needed is to wire it as you suggest. That's only 2 wires + ground.
On a Boss pedal where the jacks aren't soldered to the PCB it could be done better. One of the mono output jacks could be replaced with a stereo version and the extra leg could be wired to the other output. This way there would be no need to drill any new holes in the box.