This is a very good idea. However, how do you propose players defend their supply lines? I think having to post additional smaller armies next to the supply line would be a bit cumbersome. Nevermind the fact that small armies are vulnerable, they're basically thrown away troops.
I can imagine armies having to carry food while marching and each unit can only carry so much food. When the food runs out, morale drops, units desert and starve. Then you can use a high speed "caravan" unit to move supplies to the army. The caravan unit can be attacked by other players, but because it is so fast you won't be able to attack it with a large army (Assuming large armies are slower then small (cavalry) raiding parties).
I like your Idea best and believe it critical because It makes the map relatively smaller. The longer your supply route, the more chance someone will attack it and make your army starve. Thus limiting your effective attack range. It will force players to stay local instead of attacking people on the other side of the map.