File a small slit in the end of the slot so the card fits into it, but runs past the back. The card will run at Gen 3 x1 speed, but otherwise work properly.
Many motherboards even come with the end of the PCIe slots open for this exact purpose.
Edit: Gen 3 x1 runs at almost a full GB/s, so a 2.5Gb/s card (notice the change in size of the "B") should have more than enough bandwidth on Gen 3 x1, even at 2.5Gb/s full duplex.