So how are you opening the tank and dumping it in without stopping?
At least for me (and others I ride with) the hassle isn't worrying about running out of fuel, it is needing to stop and refuel...
Maybe a functional, albeit hack-job, solution would be to drill and tap another port into the tank which leads to another tank you could add on? Kind of like an expansion tank. As long as you sealed it so it doesn't leak, why wouldn't that work OK? It isn't something commercially viable for liability and legality reasons, but if you did it yourself why not? Disclaimer: yeah I haven't thought this through, but just trying to come up with alternatives.