Checkers assuming 737-300 you also have to consider TAT limits at SE green dot speed as well as LRC. Green dot equates to SE ceiling primarily for drift-down and terrain clearance considerations but around 250kts IAS is a closer rough figure to SE LRC. Enroute (if terrain isnt a problem) I usualy keep in mind my approxamate LRC level, which in ISA+15 to ISA+20 is about 3000 ft below the current SE green dot level. If terrain IS a problem then of course green dot level is paramount. So if you were on a sector with no terrain problems and lost one, you could just set MCT and go straight to your LRC altitude while descending at your LRC speed expected at that level (approx 250kt). This would save you fuel.
Also, if you were going say BNE-CNS in the Wet on a dark and stormey night would you realy consider coming all the way back to 210/220kt?
I dont think going below FL200 would be a problem in Oz as ATC I think would continue to provide operational control for your flight wouldnt it?