What about using a Dualie instead of 1 rear wheel. That way if one goes flat, you dont drop down to the road and can probably drive to a fix it. Maybe you dont even need the spare tire or inflation stuff, which may offset any extra cost. I imagine one on which you use two smaller sized tires, and each tire would be cheaper, to some degree. Or the two together being a little wider. I dont think it would cost a lot more and would be safer.
How that affects on traction, braking, aerodynamics, etc. ? Maybe you get a little better braking and traction if its a little wider. Anyway, makes me wonder. Could work on all 3 tires.