Get some speaker wire. Get some wire nuts. Move the receiver closer. Calibrate. Move the receiver back to the rack. :)

lippa's idea might work best. I tried using extension cables on the Audyssey mics before with mixed results.

Some day they will let you use your smart phone or tablet to do the calibrations. Plug a mic into the headphone jack or USB port. Run the calibration. Upload it to the AVR. Profit. In that order.