The plastic might tear/stretch out at the hooks or near the corners of those slots for the webbing after time with heavy loads. But if you use straps/bungies that pull up and compress the weight of the panniers to the top rail at the hooks I could see it working. How about two 3/4" webbing straps that go from the back of the panniers under the hooks all the way down the vertical plastic sections of the trash can, under the bottom, through the holes in the trash can at the front, up over the dry bag to fastek buckles at the top.
So one would hook the bag on the rail, route the 3/4" fastek buckle and webbing under and over the rail then clip onto the fastek buckles over the front. The webbing can take the place of all that hard plastic used to transmit loads to hooks on commercial panniers.