Some thoughts I had for how this could work:

/ws/2/<entity-type>?target-type=<entity-type>&target=<mbid>

e.g.

/ws/2/release-group?target-type=series&target=703650d6-0bdd-430c-961a-7cd322cdac5d

or optionally, to filter on a specific relationship by type id,

/ws/2/<entity-type>?target-type=<entity-type>&target=<mbid>&type-id=<type-uuid>

Defining the return format is a bit trickier. It would be nice to have it return e.g. a <release-group-list> like the standard browse, and it (IMO) must support all of the inc= parameters that the standard browse has (e.g. artist-credits, labels).

However

Using the <entity-list> format means that you do not get to see the relationship attributes! Which for this browse request are rather important.

A couple of ideas:
1. return a paginated <relation-list>. In this case, it might become more difficult to implement the inc= parameters, and it can't share any code with the normal entity-list or relation-list stuff. But probably workable.
2. Include the backwards version of the relation being searched for inside a relation-list in each entity. This would be a bit trickier to parse, but would fit in our current XML schema.

Any thoughts?