[btilly]

For the best gravity assist you want to have a large delta V, and you want to come in on a hyperbolic orbit that causes you to turn by 90 degrees.

This object has all of the delta V that you could want, but for an object of that mass, the hyperbolic orbit would require going through the planetoid which you can't do. And if it was dense enough that you could (for example a miniature black hole), the tidal forces during the turn would be insane.

So no, this object cannot give a decent slingshot.