I think the match is already calculated in its entirety in the 15 minutes before the match begins (the period where you can no longer submit orders). It's already over even if you watch it live.
Mr Miner states the reason this can't happen perfectly.
One way this could work, and be okay for managers who aren't online, would be to offer a change in tactics 'decision tree' at halftime. I state this full well knowing it would be probably too complicated to implement, but just for fun, it'd go like:
Starting Tactic - Look Inside
Halftime, if Look Inside is GDP'd by other team - Switch to Motion
or
Halftime, if losing game by X amount - Switch to Run n Gun, FCP
etc...