A little while ago I had the misfortune of my car (E38 740iL) toppling off the jack whilst changing my right rear wheel. What i initially thought was going to be an inconvenience turned out to be a bit more urgent, the base of the jack had actually punched a hole through the bottom of the fuel tank and fuel was pouring out of that hole (~1 litre per minute). Cutting a long story short I managed to complete changing the tyre and made tracks for my local auto shop leaving a trail of petrol behind me. The guys at Repco put me onto a product from CarGo called QuikSteel which is a putty with two component within it, you cut a suitable portion off the stick and knead it until the two parts are well combined, then jam it into the desired position, in my case the hole in my fuel tank, which still had petrol pouring out through it. After about 15 minutes the putty sets hard, but the fuel leak stopped immediately.
A great product with a variety of uses, well worth spending the money on to have ready in an emergency situation. BlueMagic