I've now compiled bcache-tools and the bcache Kernel module to test this.
bcache is not suitable for Unraid since you always have to mount /dev/bcacheX and this is simply not possible on Unraid, at least not with workarounds and a lot of tinkering...
Anyways, I've tested it with a device outside the Array, mounted it manually and I have to say, it works.
It is of course pretty cool but as said only outside of the Array or Cache pool.