Chained API Call : Membership.contact_id EntityTags
Ideally I would be using the $value.contact_id var but I can't get it to work that way. 

membership->tags (via contact_id)

What I did get working was inserting a 3rd link in the api call chain. 

membership->contact->tags

CRM.api3('Membership', 'get', {
  "sequential": 1,
  "status_id": "Current",
  "options": {"limit":0},
  "contact_id.contact_type": "Organization",
  "api.Contact.get": {"api.EntityTag.get":{}}
}).done(function(result) {
  // do something
});