For alliance there are circular messages.
Maybe group chat, where you can add that player which you want. (maybe, you can only add players which are in your alliance or on your buddy list)

But i like your proposal. ^^

I did not try this yet, but in general it should not be too much work to emulate a group chat.

a separate chat box for a group. add the userids of the players in the group into a list and send the message to every user instead of a single user.

create a new socket handler that extends the current one. when sending a group message, internally use a group chat identifier as a suffix for each message, e.g. GROUPCHAT!!!! my message.
when recieving a message, check if it uses the group chat suffix. if yes, display it in the group chat box, else in the normal user chat box.

however, the difficulty would be to recreate the group chat / remove group chat messages from the ordinary user chatbox each time the page is refreshed