craftsman makes two tools everyone should have. the telescopic magnet and one of those bendy things with the retractable claw at the end.
in your case, if the nut is indeed in the subframe (is that possible?)i would suggest somehow securing a small magnet to the end of a clothes hanger (cut the hook off and straighten it out). then find a opening for the magnet, and push it towards the lost nut. the nut should stick, allowing you to pull it out.
i lost a nut on my install on the subframe. it took like 45 min to find it so i feel your pain. mine didnt fall inside the subframe though.