It really depends on your intention.
As is, adding "par hasard" in a question asserts that you sincerely ignore whether your interlocutor can satisfy your request, that you are not asking him something with the purpose of letting him no way to say no.
Auriez-vous l'heure par hasard ?
It would be technically useless to add "par hasard" if you saw your interlucutor giving the time few seconds ago.
Saying "par hasard" asks if there is a coincidence in your favor, saying "s'il vous plaît" just asks if your interlocutor is willing to help you if he can.
But, as @Pogus says, it can be completely ironic too.
Taking the same example above, if you ask the time to somebody that just gave it and saw you watching him giving the time, he would then know that you are not asking "par hasard", you are asking him on purpose, with the conviction that he can fulfill your request. It feels like a trap.
So i would say it can be a indirect politeness by default but its sense is related to context.
If you want to be polite with no ambiguity, simply add "S'il vous plaît" imho.