If you go to the ironbutt site I believe they have links to fuel tank fabricators who specialize in motorcycles, including adding a 'spacer'.
The FJR mentioned earlier may have been *John Ryans'. His fuel tank was similarly modified.
Years ago I used a bladder cell inside the tank bag. Tubing to the non-pressurized side of the fuel return line (not a VFR). IIRC it carried a bit more than a gallon and was enough for a frequent trip @ the time. the bladder didn't need a vent. A laptop provided pressure. A hemostat was the valve..
A really long time ago I extended a bmw airhead tank by cutting the original tank straight down ~3" behind the fuel cap and within an inch of the bottom. Then horizontally back over the fuel taps, then straight down again. With another matching tank I adjusted the cuts to make the final tank ~ 3 inches longer. It worked well, and looked 'factory', but was not worth the headaches.
We each assess our risks. I would not use a fuel tank as a push bar for the next fool that rear ends me.
*If you've not heard of John Ryan You'll find information on the Iron Butt site regarding John's many record setting long distance motorcycle trips.
October 13th marks the date that John, riding his FJR, was hit from behind and killed. A rather good book was written about John by Melissa Holbrook Pierson. It is titled; "The Man Who Would Stop at Nothing". ca 2011 w.w. norton & company. Ms. Pierson also wrote; "The Perfect Vehicle". ca 1997. again w. w. Norton.