Julia, obviously there's a list as long as your arm of South African plants grown in the northern hemisphere, though as Francois pointed out above the definition of 'high altitude' might cut it down a bit. I wonder if it might help if you could share what you have already sent the authors? Also do they have a list of species in cultivation that they are looking for photos of? And are photos of garden hybrids of interest (I'm thinking of genera like Kniphofia, Crocosmia, Agapanthus and Dierama in particular)
Best, Tristan