ACS Implementing User/PhotoCollection Relationship

With ACS a User can create a PhotoCollection. Using Cloud.PhotoCollections.search they can then retrieve a list of all PhotoCollections of which they are the owner.

Using Cloud.ACLs the owner can then assign read/write acces on a global or per-user basis to each of their collections.

So UserA could create CollectionA and assign an ACL which gives UserB read/write access to it. But how can UserB query to see which PhotoCollections it has access to?

If UserB queries Cloud.PhotoCollections.search it will only return any top level PhotoCollections for which UserB is the owner.

The only way I can see would be to create a CustomObject when UserA issues this initial ACL for UserB to CollectionA. But if that is the case, then the ACL is not useful to me. Because if I define the relationship in a CustomObject (which UserB would query against to find what PhotoCollections it has access to) I could set the read/write permission for the user here.

Any advice on how to do this?

Cheers!