does anyone know if it would be possible...
does anyone know if it would be possible to put a tarp over this to make it waterproof?
It is definitely possible. Just set the tarp up more than 26 inches above the ground (height of this bivy) and put the bivy beneath it. The Snow Peak Ponta tarp is a good option or the Granite Gear white lighting tarp.
A tarp is definitely the way to go.