Is it possible to assign commanders during a siege or battle?

When an army is idle, you can assign commanders to the center and the flanks. However, if the army is already part of a siege or in a battle, it doesn't seem that you can.

Or can you?

It seems a bit silly that all my top military men are just lounging and sipping tea, while the soldiers run around like decapitated hens.

Solution 1:

I don't think it's possible to change the commanders while the troops are in battle.

There is a trick to doing this while in sieges though. Pause the game, then order army to move. You'll be able to choose commanders then. Assuming you want to keep siege the same place, cancel the move order. You can do this by having the army selected and right clicking the county they currently occupy.