No weird RAAF thingy at all - just the SIDs at Darwin are all designed for use in a radar environment. Obviously the DN3 is radar only and all of the RNAV SIDs (which is the rest of them) are not suitable for procedural control. This is because when going procedural we need you quickly on track so that we can prove lateral separation with everyone else and all of the RNAV SIDs don't have you established on track until you are far too far away from DN. This leaves us with no SID that we can issue to you. If you look at the Alice/Hobart/Launceston SIDs they are all designed to get you quickly established on outbound track.
The procedure that should be adopted by the capt. is the same they would do at any uncontrolled aerodrome at night, so yes you can 'do as you please' being responsible for your own terrain separation below MSA and also complying with the requirements to be established on track within 5nm. Also of all the procedural locations the three I mentioned before are the only ones with SIDs, so the procedure used probably was what you would get at 'any other procedural airport'.