Aristotle Pagaltzis in the rest-discuss mailing list:

In REST, when you need to provide semantics that aren’t directly available at the protocol level, your first instinct should be to expose another resource; and the way to make that all work together is to a) provide links to clients, that b) have annotations saying what semantics the client can expect that resource to expose. [...] By staying within the confines of the uniform interface and hypermedia, you can contain the spread of that special knowledge to a minimal amount of code, and you maximise the amount of accidental usefulness that clients without knowledge of your protocol can still get out of the service.