Tunnels using gre-bridge (1.3) or gretap (1.4) encapsulation are not listed via tab-completion as possible bridge member interfaces, but can be used anyway.
Appears to be same as T1779.
Minimal config using live 1.3-rc1, similar for 1.4-latest with 'gretap':
set interfaces ethernet eth0 hw-id 'aa:bb:cc:dd:ee:ff' set interfaces tunnel tun0 encapsulation 'gre-bridge' set interfaces tunnel tun0 local-ip 10.0.1.1 set interfaces tunnel tun0 remote-ip 10.0.2.1 set interfaces bridge br0 member interface eth0 set interfaces bridge br0 member interface tun0