Could it not work like Frozen Synapse? The turn won't commence until all players have clicked the 'turn' button? This would be as close as it is to the singleplayer option and encourages both active and passive multiplayer games. There could be a timeout option where if a player logs out or is inactive after a while, they will automatically take their turn. It could make for interesting multiplayer gaming, so that you could have a game on the go throughout a period of months.
Imagine playing a multiplayer game with a few friends or some strangers. In the evening you'd all play actively - logged on at the same time, weighing your options, and everyone commiting their turns when ready. The turns play out, and progress is made. You then have to go to work the next day, so you log off, and go to work the following day, whilst the other players do the same. I you can then come back after work / going out etc and you pick up your turn where you left off. If you remain away for longer than a day, the turn will eventually take place. It could the sense of an actual galaxy existing online which you can choose to participate in or not.
anyway, my two cents.