The official rule for confirmed standby is that you must keep the same routing. Does a direct flight (i.e., one that stops somewhere but keeps the same flight number) show the intermediate stop in its routing line? If it does, then standing by for the nonstop is technically not allowed. If it does not, then I suppose you would have a good argument that you are not changing the routing.

Either way, you may find an agent that will do it for you. Also, you can always go the non-confirmed standby route, if that's an option for you.