Well I think that your above example IS the non supported config, the wild leg is like 208v to ground and so would be above the 150v to ground limit, but if you use the other common low voltage 3 phase service ( 208v wye) all the lines are less than 150v to ground and would be fine.
no such luck they don't make them. I guess I could run a 480v radiant heating bulb, but I think I will just switch the transformer on and off with a thermostat and then I can plug in a cheep 120volt heater if it will not keep it warm enough ( I think that the transformer will keep it close as I only need to keep it say 35-40 degrees in there and I have it insulated. )
Nope I didn't need any 277v so I didn't put an another wire in the trench...
William....