BuddyPress.org

I’ve never done this, but…

I believe @mention uses a member’s login name – so it is unique.

I also believe you can change that so their user_nicename is used by putting:
( 'BP_ENABLE_USERNAME_COMPATIBILITY_MODE', false );
in bp-custom.php

Note: user_login, user_nicename and display_name are all the same unless changed in wp-admin or a user’s profile. But that change only applies to display_name afaik.

So if you want the functionality of @mentions, I believe you need to expose user_login.

@[name] is set via \buddypress\bp-members\bp-members-functions.php -> bp_core_get_username() which has a filter
apply_filters( 'bp_core_get_username', $username );
You could experiment with that filter and see what breaks :}