We met a guy who said he took his boat up the Lane Cove River for a bottom job. Sorry, the name of the place is lost
in the mists of time, but you might find it by Googling. Also,
there's a small slipway adjacent to the pier at Palm Beach in Pittwater, called something like Gonzales or Gonsalves. It may be that the yellow pages will be your best friend with this one.
FWIW, Mark Barry-Cotter of Bayview Slipway was very reasonable to work, in our experience.
I completely understand if you do not want to go to Pittwater for it; I'm just more familiar with what's available around there.
Good luck with it.