If you mount the transmission between two wheels, and drive those wheels with separate chains, then drive the furthest wheel with another chain off the center wheel, then chain failure would not be quite so much of a problem. Of course building a reliable chain drive system in the first place is a better idea
What you end up doing will probably depend on your design and fabrication skills, abilities, and equipment. It's really easy to set an Andy Mark shifter trans on top of the frame between wheels and run chains from there to sprockets on wheels that are mounted on dead axles, no fancy machining required.