I've never heard of a grey water switch before. I think you most likely have to go one way or the other.
When I worked in the Carribean they made these "Basements" which where not basements but rather massive water reserves. When the rains came the water would hit the roof and the down spots where designed so a little leakage filled a reservoir and when this had enough water in it the pipe would fall forward and the second down pipe would line up with the gutters rain water.
This principle was so that dust, dirt, salt and crap would first be washed off the roof. Then when the water coming was cleaner it would enter the basement container. Perhaps you could design your sump to hold more water and when there was a lot of flow the sump would fill up higher. Maybe there a second pipe hooked up to your sump pump???
Hope this helps you some. Must be freezing in the Peg? I graduated from Kildonean East back in 86 and built my first bathroom in South Saint Vital the following year....