yes, friendsunhosted and sharedstuff did it that way. it’s not ideal, and it’s maybe the biggest architectural problem of the web: the lack of a way to send a “ping”. for transactions in a tab, this is exactly the same problem as for posting comments to a blog thread. in general, from a blogosphere perspective, this is known as linkback, see for instance http://blog.michielbdejong.com/entry/four-simple-tips-for-web-page-providers-1-2.html (third chapter of that post). i think every user should run a polyglot linkback server. that way, you would have a way to alert other people in a tab that you published a new transaction.
sockethub doesn’t, by itself, provide a linkback mechanism. but it does, if you combine it with a chat server. actually you could use a mailserver instead, so that you can send messages to the other people while they are not connected to sockethub.
a big drawback of this is of course that there is no well-defined way to contact a user, other than human-readable text. so either:
- you set up a custom chat server as a centralized walled garden, and grouptabs only works properly for people who connect to that one specific server
- you define and publish the format of the ping messages you send, or use an existing standard like pubsubhubbub or WebMention. then everybody can run their own grouptabs-ping server
- you send out human-readable emails or xmpp messages with a hyperlink in them, and the user has to click on that to get to the updated tab with the added transaction