I don't know if AA is subleasing space to NWA or not, but a technicality is in order before we return to the pissing contest on who needs to answer whose question first...
Technically, the hangar is not AA's property -- it is under long term lease from the city. Ever since Eastern's collapse, most airport facility leases have been modified to stipulate that if a particular space is not required to support daily operations, AA is legally obligated to make reasonable accommodation to any carrier requesting to sublease space.
That goes for gates, ticket counter positions, offices, warehouses, ramp parking, and yes, hangars.
So, this would be the same situation as with the MCI hangar a few weeks ago -- since AA isn't using all of the bays, the city has the right to demand that AA allow access to any carrier, including NWA, if they're in need of space to support their operations, and there is no other vacant space available.