actually, you don't need to give slaves back to the sacrificers, just select all the blood hunters and press Z (caps required) and the blood slaves on all selected units will go into the lab, leaving the ones on the sacrificers.
Which would be almost as tedious since i work with 2x hunters to sacrificers. Either i have to manually put slaves on sacrificers or manually Z all my hunters. I guess it would probably be easier to take the slaves off hunters, just not by a whole lot.