I see. When you find a solution for storing the frontend form values in a json file I would be very interested in it. I could use it for my own page where people can register for a specific event and I could view the participants, then.
For testing purposes I already ported the guestbook plugin to flex-objects. But there I also would need to be able to store frontend form data (not on admin interface) as json.
Edit:
This one may help me: Json filetype in (Contact)form with flex - #3 by epb
Unfortunately it doesn’t work when adding data to a single file, but only for one dedicated file per entry.