@imalphawolf Yes, as @CotU42 mentions, this idea has been floated - even in dev streams, I believe. Alliances are designed to be shaky and, as the devs have discussed, infiltration is a part of that. Recently, I got my stolen Ashen chest commendation because I swam over to a brig and sloop anchored near each other. They’d just formed an alliance. I boarded the sloop, waved at the crew, went below deck, and hopped out the back with a chest with no one the wiser. Not possible with your idea.

@imalphawolf

Why this is not a good idea, which also can be found in the dozen of posts about it:

1. Any pirate not on your crew boarding your ship without permission is an enemy.
2. The game must not provide you with a false sense of security by UI elements.
3. Infiltration detection should not be done by a simple UI element going: he doesn't belong!
4. Learn to interact with the crew you ally with, get to know them a little.
5. Never fully trust a pirate that is not in your crew! Betrayals are a thing!

This is limiting the options of the gameplay, creates a sense of security where it shouldn't and diminishes interaction between players by replacing it with automatic UI elements. All aspects that are against the ideology and idea behind the seas.

@imalphawolf
Or you could just organize a cosmetic you all have attached. Alliances are meant to be infiltratable by other crews, as it leads to the shakiness of them.

That's one of the cons with alliances, you aren't 100% certain which ones are part of your alliance.