That would not really work for rabbits for several reasons:
It would risk damaging their hocks - especially if it was wire;
They would not be able to 'munch and poo' which is what is natural to them - a hay filled poo tray is a delight for a bun . .
any grid that was big enough to let poo through would risk trapping a paw or nail and pulling them -
Try backing the tray into a corner or raising the sides a bit (practice heights with a cardboard box and keep cutting down until its right).