"Tail-dragger" tanks are a fairly common fitment. They allow a passenger to be carried and/or increase loading capacity. On the FJR that matters because the side-cases are horrible.
I've never heard of this arrangement causing any safety issues, above and beyond those inherent in carrying 6.6 gallons between your legs above a very hot engine. Even in a rear impact the aluminum is not likely to split, and if it does you would be pushed forwards, away from any spill (besides, you'd probably have more than that to worry about if you were hit hard enough to break the tank).
One issue is that tail-dragger tanks require a fuel pump, an anti-siphon valve and the associated wiring ... another point of potential failure.