You should be able to put single phase 240v loads across the 120/240 side with no problem, up to the total capacity allowed for all single phase loads. The 3p 240v loads could go up to the total capacity for the wild leg. But how much total capacity can a utility provide if all your load is on the single phase side of a 240 center delta? It's all on one side of their distribution. If you're a really big customer, they will want to get it balanced. In theory, if the loads don't need a neutral, and don't care if the neutral is "off the line" somewhat, you could get equal sized 240v transformers and balance all those loads on all three phases even with 240 delta.
But utilities are phasing that out and want everyone to use 208/120 or 480/277. Only one utility I have ever seen offers 240/139 and only a couple offered 416/240 or 400/230. I certainly understand and agree with wanting to move away from delta. But 416/240 is not hard to do for cases where the bulk of the load is 240 (then another transformer to get 120 out of that for general use). 240/139 would require an unusual voltage secondary, but 416/240 would not.
