I found a somewhat obvious way to place guards on the scaffolding. They wont fight, but they look cool.
1. Take the torch (not glowing) (or any doodad that can placed anywhere).
2. Change model file to the unit you want.
3. Place under scaffold, then use CTRL+Page Up to raise the unit up onto the scaffolding.
Most of you probably know this, just though I'd post for those who don't.