Superhappies care about the happiness of all. Therefore humans can blackmail them by parking ships near stars in inhabited solar systems, and threatening to supernova the star in case a Superhappy ship jumps into the system, or whatever. (The detonation mechanism should probably be automated so that Superhappies can be certain humans actually go through with the detonation, instead of just making the threat.)

For humanity to maximize their influence over the future, they should immediately proceed to set up as many "booby-trapped" solar systems as they can. Then renegotiate with the Superhappies (and also spend more time thinking).

(I only had the patience to read the first page of comments, so I don't know if there was already talk of this option.)