I believe Mercedes have found a way, by creating a duct through the endplate. When DRS is closed, the flap and the plate it attaches to is in a nearly vertical position. When DRS opens, the area the flap initially covered is exposed. If this area featured an opening that lead into a duct inside the endplate, when DRS opened the high pressure above the wing would force flow through the duct. With this duct then routed through the car to the front wing, when DRS is open the FDFW would be blown and stall in unison with the rear wing. Clearly when DRS closes the duct would be closed off and the duct would stop blowing the FDFW, restoring front and rear downforce.