I honestly dont see that much of a problem in tactical battles for MP. It has been done a few times; AoW allowed you to at least watch the fight, which CAN keep you entertained (but the problem with that is, if you and your buddies are not cooperating but playing something deathmatch-like, it is a spoiler of his army). Heroes of Might and Magic V allowed you to skim through your cities and "plan ahead" and you even had a ghost to move about the map while you weren't on your turn.
I do agree that tactical battles in MP are important. They are maybe less important in random online games, but even today, there are groups of close friends that come over to LAN or play through Hamachi in their private networks just to team up against the AI and enjoy a game together. Tactical battles in this case ARE the a blast and form a good half of the fun in these kinds of games.
Personally, I have a friend who does not play CivIV or EU3 with me just for the sole fact that battles are more of a roll of a dice, rather than actual battle. I tried to tell him that he misses a lot, but alas, the battles are too crucial for my persuassion skills 
I think combination of HoMaMV and AoW style would do best. Allow the player to issue build/train orders in his cities and such and also allow him (through a notification icon to the right, for example) to jump to the battle to watch at his whim. To deal with the before-mentioned army spoiling, this feature could work only for allied players.
Hell, you can seriously add a mini game of Pong or something. As long as the player doesn't have to stare at a big ugly box saying "You are not on turn, my divine all-powerful Sovereign." or "There is a tactical battle taking place, which even someone gifted with magical clairvoyance such as you cannot see or alter, m'lord.", but has something to keep himself occupied instead, it will do.
Thinking about the tactical battles, I just realized the single biggest OH MY GOD THATS AWESOME tactical battle feature that Age of Wonders had, was the fact that battles could have multiple parties involved, if they were placed adjacent to the attacked tile. This really is the king of cooperative-tactical-battles-in-multiplayer features. If that isn't too inconvienient to implement.....you get me what I am pointing at..