Time to chat, yes. Fix your empire, no. You can't have access to your empire at all.
In a turn based multiplayer game only the player who's turn it is can do anything. If tactical were implemented then the defender or any other players with ships in that sector could move his ships during combat obviously, but nobody else.
In a simultaneous multiplayer game you only make changes to your empire during the order phase. Combat occurs during the movement phase. If tactical were implemented then the movement phase would pause for each round of combat which would be resolved by the parties involved, but nobdy else would have access to their empires to do anything.
So you see it is technically possible, but not very efficent. Of course as you say everybody would know that going in. I suppose they could chat or cruise the internet while the guys with ships fight it out.
I also suppose the simultaneous turns turns could be broken down even further. Instead of turns Lasting a month, they would be broken down into days. Instead of giving orders for the whole month, each day would tick by and you would have the option of moving ships that had movement available. No ships could move more than one space per day, but faster ships would have movement available more frequently during the month than slow ones. Build and research queues could be accessed once a month.
It wouldn't eliminate the down time, but it would space the combats out more so you didn't have them all together at once. I wouldn't want to be the one to code that beast though.
Geoschmo