Since the municipality has asserted de-facto ownership of the structure, and the lock is not protecting personal property (eg. a bicycle) this is no different than a landlord hiring a locksmith/handyperson to remove a lone padlock from a long gone tenant.
The municipality is the official agency asserting ownership of these abandoned locks as abandoned property, and thus is the correct agency to grant permission. In that case, an organizer could have city officials, lock pickers, and law enforcement meet before the event, and present to represent the owners interest of any incidental locked items (municipal owned locks and access gates.)
