Have you played any of my Chaos games?
http://www.dom3minions.com/HostedGames.htm
You can play one solo if you want to try it out. Not that its what you want, but it will give you an idea of what can be done since it mixes up everything in the game as units or commanders. Thats all done by map commands without modding.
If you wanted it to be IMPOSSIBLE to make units then that would probably have to be a mod. Im not sure how much work would be involved. Sombre is more the guru there. But even then there would be things like events dropping them in to the game. It might work if it was done on a small duelling map, or the Battle Sim map, or Arena map.