It occurrs to me that Mazda's rotary is not the only engine well suited to hydrogen fuel. It's easy to provide for a separate fuel-only intake port in the rotary, and the absence of a hot exhaust valve minimizes the preignition problem. But these advantages also accrue to the single-sleeve valve type piston engine.
Harry Ricardo's research led England's aircraft engine makers, Bristol and Rolls Royce, to abandon poppet valves in favor of the single-sleeve valve. In this layout, the entry and exiting of gases from the cylinder are controlled by the motion of a thin sleeve between the piston and the cylinder proper. The sleeve was driven at half engine speed, to give it the correct four-stroke phasing, by a kind of crank pin. The movement of the sleeve was rotary at the top and bottom of its stroke, reciprocal at mid-stroke, and a combination of the two at all other points.
Valving was accomplished when holes in the sleeve swept past holes at the top of the surrounding cylinder. There typically were multiple ports, three intake and two exhausts for example, and one of the intakes could be used to valve in a gaseous fuel, like hydrogen.
The big advantage of the sleeve-valve engine was that it eliminated the red-hot exhaust valve and thus made it possible to use higher supercharge pressures and get high specific power outputs.
Sleeve-valve engine development ended with the shift to turbo jet propulsion for aircraft. But the sleeve valve arrangement offered great advantages, and it will make a comeback if external factors, like hydrogen fuel, tip the technological balance just the least bit.