I have this working. BTW, FWIW, it seems I cannot get "fast transition" enabled with multiple SSIDs on this device, but if you don't care about that - here's the fix.
The quick version is - don't use the LuCI UI, set up your VLANs in /etc/config/network by hand and dont take the VLANs off the bridge, take them off the raw device when creating the bridge device, then create an interface to assign to the WiFi, like this:
config device
option name 'br-iot'
option type 'bridge'
list ports 'eth0.40'
config interface 'iot'
option device 'br-iot'
option proto 'none'
The above doesn't claim or associate an IP address, so the web gui will remain hidden from the members of the iot network. Do make sure you've got at least one interface bound with an IP address though, otherwise you'll lock yourself out.
Reference the above newly created iot network interface device like this in /etc/config/wireless:
config wifi-iface 'wifi2g'
option device 'wifi0'
option network 'iot'
......