Strongly oppose sorry. It is the admins responsibility to record a demo of those he bans. Its easy to circumvent or bugger up these plugins, just trying to record your own demo messes up the one the plugin was trying to record on your client. Un expected demo recording will use a lot of harddrive space on all clients pcs which can also cause problems. Admins know how to manage their demo collections and how long to keep them for, while users might not.
Responsibility for providing demos should remain with the banning admin or the player submitting the ban request if no admin was present.
Cheers for the suggestion but I feel it would cause more problems than it solves.