I got this response from Rao Dao Zao on the Hive Workshop,
Quote from Rao Dao Zao:
You can do it with actor messages, I'm quite certain.
... Don't ask me how.
I'm going to look into it.
Figured it out, knowing that is had to do with Actor Messages it was pretty easy.
"Actor - Send actor message (TextureSelectById Decal) to main actor of unit Unit"
When you are setting up the Send Actor Message function, go to the Function Tab, not Value, which is the default. Then you can set the Decal to any of the objects in the Textures tab of the Data Editor.