Commands should be entered in a conversation window (even if they are automatically send!), but since the Messenger is locked this can't be done (
edit to make it clear: MANUALLY). In other words, such a command would be useless...
Maybe a solution would be to also activate the boss protection feature which enables you to change the icon.
Or to use the ultimate Messenger Lock plugin (found here:
Ultimate Messenger Lock v1.0 released)