I don't know how difficult or easy this would be. But in the galaxy overview where you can set the P1-9 icons for yourself and your allies, wouldn't a sort of flag/message system work better?
Such as you click the "Flag" icon on the left side, and it lets you type in a short message that you can place on planets/territorys. So you or your allies can hover over the planet and see them. Such as "Blockade this planet" or "I'm attacking here next" Etc.
I'm enjoying my first Coop game right now, about 3 hours in, and this would be really useful from what I see.