A small 3rd wheel for emergency braking would work (it could even be automatically lowered when the chassis has rotated over a certain amount), or you could just drive only at slow speeds. The Segway is a diwheel as well, and people don't worry about that.
The Segway has a relatively large moment of inertia to act as counter-torque when braking - the big human body sticking out the top of it - along with gravity, where the centre of gravity of the inertial mass is well away from the axis of rotation. It also travels slowly (I think the Segway is a more viable product at the speeds it travels at than the device in this article, which IMO really needs to go faster to justify being strapped in etc.).
