I read through this whole thing and obviously I'm missing something.
OP....are you a model or the photographer? If you're the latter......forgive me but you're seriously worried about becoming physically excited when shooting a nude or scantily clad model? Again, I mean no offense but I'm guessing you're not 13 years old. I cannot for the life of me imagine that this would truly be a problem. Even if it were to happen, you're wearing clothes, you're moving around. If you can get excited to the point where you are worried that your model might notice then I guess I would say to you.....congratulations. That's quite a feat to be able to do that under those conditions. If this truly is a problem, wear tight underwear.
If you're a model and this were to happen, that would be completely normal and it is something that has happened many times before. As mentioned above, provide yourself some relief beforehand and if it still becomes a problem, I doubt very much anyone on the shoot is going to see it as a major problem. Its supposed to happen when two people get together in a sexual situation. Although, the circumstances of a photo shoot are anything but sexy, even when your posing with a nude model. She is a professional and I'm sure she has experienced this kind of thing in the past. Again, if it were to happen, it might just add to the pictures.