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