Hi Jonathan,
It's great to hear you're building a custom integration for online events!
It is currently only possible to retrieve Attendee objects via our public API, so any updates to Attendee objects have to be made via our UI. Therefore, I would recommend operating the other way around, if it is possible for you process data retrieved for your Eventbrite attendees within your app, rather than sending data from your app to Eventbrite.
Best,
Eventbrite API Team