I wish there was something like this in the game. I don't run into too many problems with the way the game places units at the start of battle, but there are some instances where a really squishy unit (newly built, wounded, tamed, etc.) is put right in the front and gets swarmed before I have a chance to move it. Another way to do this would be manual placement of units before each battle, but that could be quite tedious and your idea is much better.
