The Horizon, (and Widelux and Noblex as far as I can tell) already use variation in slit width to control exposure. On the Horizon the lens barrel only rotates at two speeds, controlling the High and Low exposure timings. Slit width is set by the Aperture control lever thus varying the amount of time the film surface is exposed to light as the barrel rotates. At the end of each exposure the slit closes completely (with an audible click) which prevents the film becoming exposed when the shutter is cocked, the film wound forward, the lens barrell returned to it's starting point and the spring re-tensioned.
Originally Posted by AgX
I'm pretty sure that introducing a "reverse swing" element would make these cameras even more expensive and complicated. And, as for the Horizon, make the beast less reliable.