A great idea indeed.
Maybe you could just have a man disguised, so the enemy AI wouldn't attack him on sight unless you order your men to shoot at him(You'd know his enemy if you couldn't click him, in this case the enemy player), therefore you'd keep him hidden so the enemy player wouldn't suspect. The only way to detect the traitor would be to have an officer in the area. If you have an officer at this camp in a tree, pretty much all approaching spies would get spotted and the AI would auto-shoot at him. It's also good cause from what I saw on MP you can't have many officers at once.
My idea for now of having an ally behind the enemy lines, is the scout, moving him round the enemy if I manage to survive and move bush to bush behind em' and dynamite em or something.
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