Customisable "You cannot post here" message

Having "You do not have permission to send messages in this channel" as the default, why not allow a custom message to be shown in the text-entry area when there is a reason someone is unable to post in a text channel.

Practical examples could include:

• This is a server-wide announcements channel; only administrators can post here.
• Please introduce yourself in the #introductions channel first.
• Only members with the [ROLE_NAME] role can post here.