does anyone have a recommendation for how to best apply a featherboard to the fence to hold a workpiece DOWN when making a dado cut; 

The ones I've seen require drilling into the fence body. Alternatively, you could build an auxiliary fence to fit over the SawStop fence with a featherboard attached to it. The aux fence would also be useful as a tall fence for cutting angles with the piece on it's end. Check out the Fine Woodworking article on Bob VanDyke's fence system.

https://www.finewoodworking.com/membership/pdf/27802/011231056.pdf