I don't think the issue is the use of the fuel bladder, I think the safety issue is strapping said fuel bladder to your back... if you are thrown from your bike, the fuel line could detach, which leads to the fireball scenario mentioned previously making a bad situation (thrown from bike) significantly worse (human fireball thrown from bike).
And you said the purpose of devising an alternate gas tank is you are looking for a clean looking bike... what's "clean looking" about hoses leading from the carb to a bag on your back?