The requirement to provide additional protection by means of a 30mA RCD applies to socket-outlets for use by ordinary persons that are intended for general use.
The exception is rather meaningless, as it requires that the socket-outlets that are intended for general use to be specifically labeled or otherwise identified as being for the connection of a particular item of equipment.
If a socket-outlet is intended for the connection of a particular item of equipment, then it is not for general use and the exception would not apply.

No, use of a plug in RCD would not be an acceptable method of providing RCD protection for a socket-outlet. Such would only provide protection for the item of equipment it is attached to.

Thanks for all your responses so far on this.