The "cable" you need would be a USB cable. That's the only way anything other than triggering the shutter could be controlled. Unfortunately, I haven't found any small device that could be a USB master. Everything such as iThings and so on are USB peripherals. The smallest you might get, is a netbook.
If anyone knows of a handheld device into which USB peripherals are plugged, then let us know! That device would be a USB master and some kind of camera control s/w might be written for it.
Andy
(BTW, Canon engineers have already designed a mirror lockup button. And lots of other things you want. It's the marketing department, just as in Nikon and others, that tiers the features into different levels of bodies. Otherwise, you'd never upgrade would you ? )