Yes fair enough, probably illegal in a lot of countries.
What would you do in the case of a medevac and you were in the situation that you needed that extra fuel?
All I am trying to advise is if you are faced with the case of having to, do it properly. Done it on a lot of occasions over the years in Africa. Oh and BTW empty fuel containers are more dangerous than full containers, in a lot of circumstances. If you do have fumes in the cockpit, even flicking a switch could ignite the mixture.