Personally, when I think "multiplayer roguelike" I don't think of an MMO. (Probably because I hate MMOs.)
My first thought is multiplayer like Civalization. It's turn-based. While each unit has few choices, each player controls a lot of units at the end.
So: a small group of friends play together.
Now consider: there is no actual requirement to keep the turns in sync across all players except when they're within line-of-sight. If I can't see you the difference of you going one turn or 10 to my one turn is meaningless.
So, when players are within line-of-sight, they need to wait for the other players to finish their turns. Talking to each other is still a free action, of course.
Next, more squelching of boring things.
If the group is moving as a unit, one player is given control and everybody moves as a single unit until enemies are encountered. This is simply squelching manual follow commands.
If a player is on auto-explore, their next command is handled automatically by the system so there is zero delay. If every one in the party is on auto-explore together, they should fan out, explore the map, then converge on the first monster found. (Given that talking is free and the first thing a person in their party would do is say, "Hey, there's something I need help with over here!")
So, yeah... the only time you should be waiting for other players is when you're near each other and monsters are around -- or you're near each other and playing non-cooperatively.
Then again, I see multiplayer as an extension of single-player multiple-character. We see those sorts of games in RPGs all the time, but they're rare with roguelikes.
Cheers,
Steven Black