I would dearly love to see it. Basically we only really need two things

(a) A way to convert campaign map info into battle map info.

(b) A way to easily modify campaign map info to take into account battles results.

At least in MTW (and I assume in rtw) You could use the console commands to switch from one faction to another.

You could start up a campaign as normal, and just have each person play out their turn, using the console to switch factions as needed.

If your factions wound up fighting, you could fight a custom battle based on the info from the campaign map (thats where what we need (a) comes in)

Then you just need a result to put those changes back into the campaign map (b).