On my router with KeeneticOS 5 btrfs modules are available, but are not loaded by default, and can't be through the WebUI.
With OPKG enabled, you can ssh into the router and load the modules using
opkg install kmod
insmod /lib/modules/$(uname -r)/xor.ko
insmod /lib/modules/$(uname -r)/raid6_pq.ko
insmod /lib/modules/$(uname -r)/btrfs.ko
The rest you can figure out by yourself, imma use it for NFS.
I wish I knew about this before I compiled the module myself...