I love the idea of having some dual-ported RAM for a screen, as you say, very retro.
One major advantage to this is the fact that you can swap pages in an instant, that means not having to watch the crappy redraws as that's done on another page then swapped in. (assuming of course that the subsequent display of that RAM is fast)
Unfortunately I don't have the time to design a board for the Mega, I am however tinkering with a Due-style board that will have external memory, I may look at allowing for such things.
Rob