I got the impression it was similar guidelines to cosplayers at an anime convention - if people are clearly in costume they most likely won't mind you taking photos of them, but they'd probably prefer that you ask first so they can pose and give you a good picture
Don't be afraid to ask though, all the people I asked to take photos of on that bridge were very obliging and nice. Apparently the cosplayers mainly come out on a Sunday though, so remember to go on the right day!
I saw more tourists in Harajuku than anywhere else I went in Japan, so I'd bet the regular cosplayers are very much used to tourists who can't speak Japanese. If you hold up your camera and just say 'okay?' they'll probably figure out what you're after.