With OpenWRT you can manage your router configuration with the tool uci (Unified Configuration Interface).

I created an additional interface where the SSID contains the final coordinates of the Geocache. This interface is protected with WPA2 with an random passphrase and is not linked to the rest of the network. Nobody needs to establish a connection. You only have to find the name of the WIFI.

# toggle interface #2
uci set wireless.@wifi-iface[2].disabled="$1"
# save changes
uci commit wireless
# restart wireless interface
wifi

Toggling and restarting the wireless interface stales all wireless connections for a fraction of a second. But most of the times the effect isn't even noticeable.