I had a recipe back in the day for doing this by importing mailman-generated mbox archives into fudforum. It worked incredibly well, just as you describe. A quick google search turmed up an oldish stackoverflow topic about it that explains it reasonably well.
I also am very interested in the staged users concept - is this discussed anywhere in any detail already on meta? Saw a mention of it in the below topic but when I tested it a few weeks back I saw no sign of staged users or an explanation of how it would work.
Seems to me that, if staged accounts were to work, then we’re not far away from making this happen. w00t!