Not sure how it would be possible to PBEM...
Just another follow up...I'm seven turns into a PBEM game using VASSAL. The first six turns took up 41 emails. The VASSAL module is fabulous with one minor problem: the files are huge, averaging over 650 Kb. With high speed internet that's no big deal, but if you're on dial up it might be a problem.
Otherwise it works really well. I'm really enjoying the game, despite a terrible blunder that cost me six ships sunk out of the Brest fleet.