For interested parties, you don't have to use the control file. Simply pointing the AP (FTP or TFTP) to the actual .img on a server will work, though it does claim that it fails. Reboot the AP, and it will work.
The local option is gained with 9.1.2. Also worth noting- though the APs do not have an upgrade path, dragging them through several GAs at once can lead to them defaulting. If they don't come back, check DHCP.