I don't think you should depend on being able to take an extra bag on any flight in this way.

No matter what rules the airline might have, there would be serious security issues surrounding a bag going on a plane without an accompanying passenger.

You might get away with in nine times out of ten, but one day security will realise that they have more bags than they have passengers, the plane will be emptied, the bomb squad called out, and if they work out whose bag has caused it all..........!

Bite the bullet and pay the extra bag fee!

Ian