The guest networks could be enabled in ip-client mode.
The DHCP server is not working on guest networks. You could:
Change the bridging: I moved the guest1 interface to the lan bridge to make it work. It means the guest is not really a guest, but just a 2nd AP, which is actually what I wanted:
$ brctl show bridge name bridge id STP enabled interfaces lan 8000.bc05439f6953 no eth0 ath0 guest 8000.bc05439f6953 no guest1 hotspot 8000.bc05439f6953 no
$ brctl delif guest guest1
$ brctl addif lan guest1
$ brctl show bridge name bridge id STP enabled interfaces lan 8000.bc05439f6953 no eth0 ath0 guest1 guest 8000.bc05439f6953 no hotspot 8000.bc05439f6953 no