I've done it with messages sent to my ally during group battles. Sending things like, "I'll flank him from thse woods etc. etc." over the main board while sending him my real message secretly. At first my partner doesn't know what's really going on so I usually get stuff back like, "Don't tell them what we're doing!!" and I usuallyreply with "What? How do you send private messages?" over the chat, which makes things look authentic until my partner picks up on what I'm doing.
Usually the enemy dispatches some part of its army to keep guard on whatever false info they think they've intercepted due to my "ineptitude"....