This was a thought I also had. An HD recording of the message stream, with enough of the surrounding environment to make it look / demonstrate its "liveness" and context.
Rather than trying to deliver individual fragments to all the contributors, see if the ads/messages are clocked tightly enough that you can simply post one or several videos somewhere (YouTube, Vimeo?) and then provide contributors the timestamp of the start of their particular ad/message.
Possible, the ad/message stream videos may take on a life or their own. Two results for the price of one, as it were.
P.S. Beware of liability, though. At your price point, that's a lot of content to vet. And you don't have just obscene content to worry about, but possibly also trademark infringement, accusations of hate speech, etc., etc. Some of which might be hard to catch in a cursory examination. (OTOH, one brief display might reduce the change of facing legal action. IANAL and all that.)