I'm probably going against the grain here, but I think adding more attributes to RSS is a mistake. It's supposed to be about simple syndication. In the bank account example, rss simply isn't suitable and trying to make it suitable is a hack. How about using RSS to notify a customer of an event, but have the actual content of the event stored somewhere else?

In other words, use RSS as a notification service, and have the link attribute point to somewhere that contains the proprietary format. So, anyone can be notified of the event, if you want to get more details, use an adapter that's suitable for reading that particular format.