I hereby suggest message items.
How about a readable holodisc or some old-fashioned sheet of paper?
To leave a message, put the item in your active slot and enter your message in the chat box with a tag, like:
"m/Hello Chief, i borrowed some 5mm ammo from the box. Minigunner."
The message will be left in the box so that Chief can read it.
To read the message, Chief puts it in his active slot and uses it. The message appears then in the text window.
Chief can also delete the message, using a appropriate tag to use the holodisc again.
What do you think about it, is it possible to realise?