I suppose a "per-network, per-SSID" bandwidth restriction is not so easy to implement as a per-client restriction, given the bandwidth usage can be distributed across multiple or many access points and in a bridged environment with an external gateway, there is no single point to control the bandwidth. There is obviously some sort of "invisible virtual controller" and for "NATed" networks one could use the "virtual gateway" to control the bandwidth there - but not for bridged environments, where traffic would flow directly from individual access points to a gateway. In my opinion, if you want to implement this - I'd use a separate VLAN and limit the bandwidth at the gateway router.