OK, so bit more testing, and if you set up a bridge on the WAN interface, it works, BUT it makes all addresses allocated by connecting to the YUN wireless in the same range as the router you are connected to over ethernet. (I guess routing fails if you unplug the internet router/gateway)
so if your router on YUN ethernet port is 192.168.3.1 then any connection over wireless to YUN gets an address in that range, (i.e. 192.168.3.3) presumably allocated by the router as it shows up in the router table.
now if you set up the Bridge on the LAN interface, all kinds of weirdness, the ethernet connected router showed 192.168.240.1 in the router table, and then lost it and other stuff.... one oddity was that youd get the luci please wait screen, but that was all.... ssh into yun would lock rather than quick fail as well... also you could get to google if I recall correctly... but not luci...
so I'm sure its possible to have the YUN allocating wireless adresses in the 192.168.240.xxx range and bridge/route all those to an ethernet connected router on another subnet... (i.e. 192.168.3.1) perhaps its also firewall settings???
any help appreciated.