Hmmm... just read How to flash LEDE onto Buffalo WHR-1166D? and just as an experiment I tried booting the WHR-600D with the raminitfs Robin Panda built for Buffalo WHR-1166D and it boots!
https://www.dropbox.com/s/b6t5dgxcrcsz8no/20181002_OpenWrtForWHR-1166DAndapandaBuildBootlog.txt?dl=0
However the WAN interface does not seem to work.
Any ideas for getting further with this?