I don't intend this to be an exhaustive essay, and there's tons of Plato's dialogues that I've skipped. Hopefully I won't be completely laughed off the GHMB for writing this, but anyway (NB. quotes not in any particular order):
Let's see now....Plato's Critias....
"this felicitous island under the sun"
- Equatorial location? The sun passes directly overhead.
"There was an [island/landmass] beyond the Pillars of Hercules... larger than Libya and Asia [Minor] put together. This [island/landmass] was the way to other islands; and from these you might pass to the opposite continent, which encompasses the true Ocean"
- Could it be possible that the Greeks could have substituted the Strait Malacca for that of Gibraltar? Were not the Greeks much more recent arrivals in the Mediterranean area than the Egyptians? Also, we know that Sundaland was one of the largest continuous pieces of territory inundated at the end of the Ice Age. To all intents and purposes it would have given the impression of being 'larger than Libya and Asia'. The 'way to the other islands' could be a reference to Oceania, including, of course, the enlarged 'Sahul', greater Australia, which would have combined Oz, Tasmania and New Guinea. Also, Plato (or rather, Solon's priestly guide) could have had the myriad Polynesian islands in mind too. Finally, of course, 'the opposite continent' must be a reference to the Americas.
"There were a great number of elephants in the [island/landmass], for there was ample provision of food for all sorts of animals... including for the animal which is the largest and the most voracious of all."
- Maybe it's my Indian origins (I was born in Kerala after all), but the very mention of Elephants immediately brings either Southern/Southeast Asia or Africa to mind! Although I have to concede, Africa would include the Atlantic seaboard of Morocco, and Mastodons were around in Florida say 13,000 years ago. But Plato makes it clear conditions were lush enough to support a large population of pachyderms!
"The entire circuit of the wall, which went round the outermost zone, they covered with a coating of brass, and the circuit of the next wall they coated with tin, and the third, which encompassed the citadel, flashed with the red light of orichalcum."
- I wish not to be drawn on what exactly ''orichalc" was, but the mention of walls being coated in tin, suggests the Atlanteans had a lot of it - and is not modern Indonesia one of the largest, if not the largest, source of Tin today. Surely I'm not building a 'man of tin' here! (Geddit?)
"The whole country was said by him to be very lofty and precipitous on the side of the sea, but the country immediately about and surrounding the city was a level plain, itself surrounded by mountains which descended towards the sea"
- Sundaland, as it was, would have had high mountains to the south, in Java and Sumatra, and to the east, in Borneo, all these regions bordering the main Ocean, but the central portion, now underwater, would indeed have been a flat plain.
"Further inland, likewise, straight canals of a hundred feet in width were cut from it through the plain, and again let off into the ditch leading to the sea"
- Dr. Stephen Oppenheimer, in his Eden in the East (1998) suggests that the first *visible* signs of humans undertaking complex irrigation were indeed in SE Asia (I don't have the book handy right this minute, but I can find the exact cite later if you want).
"Twice in the year they gathered the fruits of the earth-in winter having the benefit of the rains of heaven, and in summer the water which the land supplied by introducing streams from the canals."
- This quote suggests two things, confirming my first quote above: a warm, tropical climate and a seasonal monsoon system. Asia, surely!
As, I said, I have missed out loads, maybe I'll insert other quotes in response to comments in this thread.
My own 'theory' is that Plato could have borrowed a much earlier tale orginating in Egypt, maybe even pre-dating Egypt. Also, I think that there could have been many locations of Ice Age civilization, eg. The floor of the Gulf of Persia could have provided the ante-diluvian ancestors of the Sumerians with a viable 'refugium', as did the continental shelf off NW India provide a primordial homeland for India's so-called 'Aryans'. And I think they did have close cultural and commercial contact with one another, and the hypothetical Sundaland civilization suggested by both Plato and Oppenheimer (although Stephen does overstate SE Asia's importance slightly I feel).