but https://github.com/anaelorlinski/OpenWrt-NanoPi-R2S-R4S-Builds has builds that work for these too.
FriendlyWRT aren't the best at upstreaming support for their systems and their hack job of modifying the r2s into a r2c profile afaik has never been properly upstreamed, thus why separate providers have been better to use than openwrt master.
:edit: that being said a wiki page would be good as at least info and threads can be linked on the pages like I did for the r4s page. I'll try take a look in the new year at some more wiki editing.