attaching seatpost acessories to rear of racks?
One possibility is to use extra rubber shims to use the seatpost attaching mechanism to attach to the stay of your rack, like so:



You might be able to use something like the Low down light mount bolted through that hole to give you a little bit of post hanging from the hole and use that with the seatpost mount...

You might be able to mount the hardware without the seatpost mechanism to the hole of an L-bracket mounted in that hole...

With racks with a more serious light mounting bracket instead of just a horizontal hole, I've found many light can clip directly to the bracket with no extra hardware at all, like so: