i believe there is a '/nick2' command which replaced the 'says' part of your nickname. but otherwise i can't think of a plugin.
and about the other request. im sorry that there was no other solution. im sure if you you could create a quicktext. that changed your nick to a white coloured one. *making it sort of invisible. and say what you need to say. eg (!N) Died cos i killed him. or whatever you do in your RP. then change your nick back to the original state.
im sorry about the lack of plugin for this though