Bring the rule book up to the officials. I've seen many teams use their bumpers to line up ubertubes, and I have yet to find a rule that goes against this. If they say you can't do it, ask why. You have the right to know why they aren't allowing you to place your ubertube on your bumper! If they give any legit reason why that is in the rule book, then you have to listen to them. Otherwise, I don't see why you can't have it on your robot when the match starts.