The situation you'd run into with the 208V to Neutral would be that standard 1 pole C/B's would not be rated for the voltage, as they're rated at 120V If you were to use a C/B with a higher rating (277V), or use a fuse with a 250V rating, then I don't think that there's anything to prohibit it in the Code.
Thanks for the replies. I was not as clear as I could have been. I am asking about single phase 240 using the high leg with either 'A' or 'C'. I was aware of the 208 and breaker rating mentioned. In my area it is an 'understood rule' that only 3ph. goes on the high leg. Transformers and services are sized accordingly. Is it a code issue or just a local thing?
If you think about it, a three phase load is made up of three single phase loads that work together to make advantage of the 120 degree phasing of the source. Each load of a three phase load, taken by itself, is indistinguishable from a single phase load.
Given that phase B is the high leg, adding 240 volt 2-wire loads to A-B and B-C will shift load off the A-C 120/240 side of the delta.
Nothing in the NEC encourages or discourages this placement of load on a 120/240 volt 4-wire delta.
The first thing I would wonder is what the local PoCo's method is for sizing the transformer(s) that supply phase B, the high leg? Do they put the smallest possible can(s) on the pole?
I am not sure if this addresses you're Question specificly, atricle 240-85 mentions breakers with a slash designation 120/240 cannot be used between the high leg and another phase (a-b)you need a breaker with a straight 240v rating.
Since sometimes people take code rules and put them where they don't belong (Ya know, I remember the code sez....) someone may be confusing 455-9 for phase convertors where it says single phase loads are not to be put on the derived phase, but this has nothing to do with delta high legs (which are also getting pretty rare).