I'm just thinking out loud here, not suggesting that Ethan scrap his clever IMAP sync script. But I wonder if syncing to
Backpack would make sense. They have a
well-documented API, and the app itself offers functionality beyond the scope of OmniFocus that's useful for capture, reference, and GTD in general. At a glance, the API interface seems like it would be a lot less chatty than IMAP, and adding/completing tasks would be more straightforward.
Of course, that leaves us back where we started: slow (at least with EDGE) online-only lists. But it's hosted.