Configuring multiple VLANs on batman mesh on DSA-based (i.e. newer) Openwrt firmware turns out to be trivial

No docs yet on wiki - https://openwrt.org/docs/guide-user/base-system/basic-networking#switch_configuration_dsabridge-vlan - just says 'TO DO'.

But all you need to do in LuCi is go to Network -> Interfaces -> Devices -> Configure br-lan -> Bridge VLAN filtering -> Enable VLAN filtering -> define your VLANs -> then for 'bat0', tag the VLANs that you want the data of which to flow through your mesh.

Simple as that. Of course you also need to define your network interfaces (one per VLAN), and remember to attach br-lan.1, br-lan.2, etc (where 1, 2.... are actually your VLAN IDs) to each of your network interfaces.