First of all you're welcome for the incredible idea. You get it for free.
I was looking into the custom one pros and after trying them out in a store, it's incredible what the size of a little hole in the bottom can do to the quality of the sound. Really stunning change! And since so much of headphone preference is your own personal tastes, it got me to thinking...
I envision a headphone that has something like a threaded back, so the entire back housing could unscrew / snap off / hinge off to turn closed cans into open ones. After hearing some open vs closed cans in a similar price range I'm sold on open being nicer to listen to. The sound doesn't bounce around and reflect inside an open back so you don't perceive the sound as being inside something.
The obvious benefit is portability. Open listening at home, closed listening on the bus / plane / at the office.
I've searched for things like optional open back, interchangeable open back, blah blah, but nothing has come up. Is anyone aware of a mod or a headphone that would allow you to do this?