I’ll throw out another solution just in case anyone is interested.

(m/rewrite db
  {:items [{:id !ids :labels (m/or [!labels ..!n]
                                   (m/let [!n 0]))} ..!m]
   :labels ?labels}
  {:items
   [{:id !ids :labels [(m/cata [!labels ?labels]) ..!n]} ..!m]}

  [?label (m/scan {:id ?label :name ?name})]
  ?name)