This is not exactly the same, because I met these people on a nude beach when we were already all nude. I explained that I’d like their help with something (before saying what it was) and it’s a bit awkward, and stressed that it’s totally ok for them to say no if they’re not comfortable with it. Then I mentioned the website and said it was not a sexual thing, but more of a challenge to be more comfortable and confident in our own bodies. I also stressed that the other person would not have to be nude themselves.
You could try something like that. But be prepared for the person to say no and to accept that. And also be prepared for the possibility that this person may no longer feel comfortable around you. In my situation it wasn’t as much of a big deal because I didn’t have any expectation of ever running into these people again. But if these are people on your regular route, that’s different.