low data mode could be provided by a relay that forwards requests and a client that has a "low data mode" relay setting that designates a primary and backup low data relay that does this fetch-proxying for you

also if there was some kind of synchrony between relay and client, like, client could send a unique id with its reqs and the relay would not resend events it has sent previously - this requires the client to also follow some kind of common rule set about when to evict events from its cache