Looking at the code, it looks like it creates a new ext4 "incompatible feature". That is, you will be able to specify it using `mke2fs -O <whatever>`, but then you will not be able to mount this filesystem in an older kernel, not even read-only, as it is marked as *incompatible*.
Now, I don't know if it will be enabled by default or if it will be changeable with `tune2fs`... more code reading required.