There's no issue with 2-pole circuits ... those will be 240v, no matter what buss they come from. Because of the difference in transformer sizes, a conscientious sparky would make sure to draw his 240 single-phase loads from "A" and "C" legs - but I don't think that's a requirement of any code.
I suppose one could connect a dryer to "A" and "B", but you would have to make sure that the dryer's 120V motor sees the "A" phase, and not the "B" phase. And that would be a problem when the dryer is replaced years from now. 240V only baseboard electric heaters would not have that problem, and should work fine. Another issue would be that the power company would probably perfer similar 240V loads not be across both transfomers, but just be across the actual high leg transformer. Any way for a sparky to know which pair of wires to select, for in this example "A" to "B" or "B" to "C", to only load the high leg transformer? "A" to "C" would be obvious, but that's boring...
