It seems to be possible to lock the BSSID in Router mode, but not in WDS mode on my Flint using the latest v4.2 Snapshot software, is there somewhere an option to do this, or could it please be added? It would be very useful.
Maybe you can try to bind the BSSID by editing the interface in LuCI → wireless after the device is in the WDS.
We have plans to optimise this part of WDS in the future, but not in the coming release.