I'm Chief Architect of Vizrt's Online division. I usually go by my nickname of Mogsie, and I freely admit that my knowledge of REST changes over time. The last five or so years have been spent implementing a hypermedia based set of resources along with a hypermedia based client. I gave a talk at JavaZone 2009 together with Erlend Hamnaberg and I did the same talk at REST Fest 2010; videos are available on-line. Erlend has since left the company, but the fruits of my (and my co-workers') labour are publicly available in the form of a few media types:
application/vnd.vizrt.payload+xml to describe modifiable fields and values and
application/vnd.vizrt.model+xml that describe constraints of such fields and values. We've also accumulated a set of link relations we use and of course we use Atom feeds to convey lists of stuff, and OpenSearch documents to convey searchable stuff. The curious may browse our documentation at http://documentation.vizrt.com/ece-integration-guide/5.6/introduction.html (the Howto section is a useful starter). No, we don't have a sandbox available (yet).
At REST Fest, I have the honour of providing food for thought at the [Hack day](Hack Day), where we'll hopefully explode in a cacophony of agents when the day is over.
I'll be pitching some ideas I have for clients, and perhaps something to do with machine-to-machine hypermedia.