There are actually a couple options out there - Alice, and SwitchLens! I've not used either of them, but both of them are apparently decent (but not world changing). I have thought about grabbing them, but I lack m4/3 lenses which would be perfect for this.
I have an Olympus E-M5 II with some nice m4/3 lenses, like the 12-40 F2.8. Once you add optics like that, the size difference between a dedicated camera and the Alice or SwitchLens is completely negligible.
wow, switchlens looks really cool. It would be amazing if there was a Canon EF-compatible mount and full frame sensor option as well. I suppose integrating other mounts and sensor sizes is straightforward once the platform is established.