My SE told me about a MIB to enable/disable an SSID, so you could just run this in a cronjob:
snmpset -v2c -c private ewc.host.name .1.3.6.1.4.1.4329.15.3.3.4.4.1.7.101 i: 2101 is the SNMP ID for that SSID (which you can find by snmpwalking .1.3.6.1.4.1.4329.15.3.3.4.4.1.4), and i: 2 disables, i: 1 enables.