Is it legal to install a 208v photo cell so it switches one side of the 2 phases? I would think running it through a 2 or 4 pole contactor to switch both sides would be better. I found one done this way and think they had trouble with it, (not to mention that one side would still be hot if you were working on it). It was for some parking lot lights. I put a contactor controlled by a photo cell on it.
The Golden Rule - "The man with the gold makes the rule"
My first guess was it might be OK if you had a 2 pole disconnect in front of the photocell but I don't see the language that supports it. I know you can have some types of equipment that allow this for control only (with a real disconnect present) but "lamp holders" and ED lighting does not seem to be included. If the installation was such that it was clear that the future maintainer could relamp safely I might be softer on this ruling.
I've seen plenty of HPS & Metal Halide floods & wallpacks that were supplied by 208 volt or 240 branch circuits and were individually controlled by a photocell. Since these fixtures came from the factory that way and are UL listed, I think the second paragraph of 90.7 would allow a factory installed photocell to switch just one energized conductor.
Of course, if you're the AHJ, it is up to you to approve equipment, listed or not.110.2
Few things are harder to put up with than the annoyance of a good example.
A lot of things go on inside listed products that we would not allow in field assembled equipment. I suppose it would really come down to your AHJ's definition of "switching device". If you called the PC something else and there was a real 2 pole switch present for safe relamping, I would not be all that concerned.