Sounds big and will require a lot of administration from the (Impartial) God Team, but does sound like a good idea. I've had a think about this and can see it being highly political too as all the commanders/Generals will be clamoring for reinforcements from their finite pool of manpower and materials available.
Favouritism and victory points (from winning engagements) will help secure Commanders their reinforcements over other Commanders. Other factors would play a part too, such as the theater of operations (you're hardly going to prioritse reinforcements to a sideshow are you?!)
The world map would have to be carefully thought out, with them seperated into theaters of operations with objectives in each.
Trial this idea with just a theatre first and see how it goes. This will bring to light any problems in God Team's impartiality, collating results of actions, timing attacks using several players/Commanders on a theatre front, etc. This game would probably need a forum all of it's own too!