Adobe InContext Editing (ICE) can do this. But unfortunately, it only works with Dreamweaver CS4. I just wrote a review of ICE. It is pretty cool and uses named DIV tags, something I am sure Xara/John could work out.

The webmaster determines the level of changes the client can make, from simple text changes and updates, to styles, uploading photos and adding hyperlinks.

Xara already has the technology in Xara OnLine Site Updater. And they have offered this service for years.