Once in a while I get a call from someone who is trying to rent a harp for a day so their cousin from across the country can play at their wedding. I generally pass these on to local harpists, who I tell to charge your normal gig rate. Then you transport the instrument, wait while the harp is being used for the ceremony, and then transport it home again.
For this photo shoot, I’d advise the same thing. Charge whatever you would normally charge for a gig that is going to eat up the better part of a day, bring the harp to the location, wait, and watch, as the photo shoot is being done, then bring the harp home again. That way, the harp is never out of your sight.