Why?

Unless you were sitting on top of it -- (not very safe) , you would have CG issues anywhere else I'm afraid...
 
NO

There a number of 7's with full leading edge tanks plus wingtip tanks. Now of course one has to be very careful taxiing with that lot out there but keeping the fuel OUT of the cockpit, and no W&B issues..well, its the way to go.

I even thought of Making a second tank on my flying 7a..its just hard to tear apart a flying airplane..:)

Frank
 
I've seen a 12 gallon ferry tank in the baggage area of s 6, I've heard of a long range ferry tank that sat in the right seat or a 7( in place of passenger).
And know a guy who found enough space in an 8 to fly all the way around the world. He put it everywhere except for more in the wings. He flew some long legs.

personaly... my bladder isn't that big