I'm diggin deep here but i think i would work, only if you have two available inputs though, but plug the 1/2 outputs (with the monitor mix) into two inputs (let's say 7/8), and set the outputs of 3/4 and 5/6 to be 7/8 in. Again if you are using all 8 ins for recording then it won't work. Or if you have any kind of A/D converter and you can set the s/pdif out to monitor mix.
I don't know though, or some kind of splitter for out 1/2