You should be able to see this working (creating the collection) by using PUT. The documentation is currently misleading cause we're not supporting POST for association resources at all. I'll use this ticket to add this functionality
That's fixed for Gosling and the maintenance branch for Fowler. You should actually be able to get this working out of the box by using PATCH instead of POST. We chose this as the method to use as it resembles the "partial update" semantics more than POST does. Although you're interacting with a collection, you don't use the resource to create new resources but rather to augment the existing one.
That said, I took the chance to revamp the internals of the checks for supported HTTP methods. POST is now allowed on collection properties and basically resembles what PATCH does