I've done exactly this before. It works really well. For endpoints that support it, you can put a subscription channel id in the normal REST resource. If the client sees the pubsub channel, and understands what to do with it, it can subscribe for updates via websockets; if it cant/chooses not to, if can just refresh the resource via standard http GET.
What's especially nice is Websockets have their own url type (ws://) you can just include a link to the Websocket's endpoint for the resource for the API client to follow. Reductions in the amount of custom stuff that has to be done is always good :)
