Good suggestions. How about retightening the nut.Then drill and tap a hole in the center of the bolt. The screw head must be small enough for the nut to pass over it. Then make an eye using a length of thin mechanics wire and fasten the loop under the small screw and tighten the screw. Now you can loosen and remove the nut by holding the bolt with a screwdriver as mentioned above. Now let the bolt slide down the hole until it clears the bottom and move it to the side of the hole just far enough so you can fit a very small drill down the hole to the bottom and drill a hole through that. This will give you the location of the hole on the subfloor and once you have that just drill it bigger so you can drop the bolt by using the mechanics wire through the bottom hole. Just be sure you don't drill into the propane tank.
