I'd like to take traffic on a particular wlan and use a gretap tunnel to send it elsewhere at layer 2. So dhcp etc would be handled by the device terminating the gre tunnel over the WAN. I also need to tag the outgoing layer 2 inner header with a VLAN tag. So the wireless client talks with no VLAN tag but one is added by the router at ecapsulation and removed on return traffic to the client. This is basically how controller based APs work.
This is what I came up with just a cursory understanding of openWRT but I'm not seeing GRE encapped packets on tcpdumps of any of the interfaces.
config interface 'gre1' option proto 'gretap' option peeraddr '184.108.40.206' option delegate '0' option mtu 1558 config interface 'br1' option type 'bridge' option stp '0' option ifname 'wlan2 @gre1.100' option delegate '0'