Its perfectly legal for the photographer to fly the machine never mind operate the fuel tank selector.
After partners have done the safety pilot course which I might add is a bit weird for the instructor because you teach it in the LHS they are encouraged to fly the plane and practise while flying with thier partner.
Its no great shakes operating a PA28 from the RHS. Instructors round the world do it day in day out. We don't get given any special training during the FIC in fact mostly FIC's are done in C150's. All you do is give the person in the LHS a brief on the ground when they get in, about what you want them to do and let them have a couple of shots with the engine off. The swapping over during the power checks confirms that they know what to do.
And to the orginal poster don't let the "experence" get the better of you. I have seen 1000 hour plus instructors scare themselves pooless doing a bit of adhock photography when they got too focused on the photography.