Shellharbour used to be in the old Jim Beam Cup (RMC), but once the Dragons funding left the club after their jump to NSW Cup, they went back to the Illawarra comp. The only clubs big enough down there would be Collegians or Wests Illawarra.
As for Sutherland, I doubt any of the clubs in that comp have the $$$ to run a team in that comp. None of them have licensed club backing which would make it hard to attract semi-pro players. Sutherland-Loftus looked like a possibility for SS a few years ago, but that died off once money came up.