Comparators do not recognize inventory of storage minecarts

As Comparators can now receive redstone signal from containers based on inventory capacity, and since Hoppers work with storage minecarts, I thought I could create an automatic station that not only unloads carts, but then sends them away when emptied.

Tested using storage carts on all types of rails, and off sitting on the ground. Carts were initially tested completely full at first, then tested again halfway full.

EDIT: After writing I thought of the fact that maybe this functionality hadn't been added yet, but since hoppers work, why wouldn't comparators?