As I see it, the commentbox would only be visible to three parties:

1.) The person who was barred.
2.) The person who did the barring.
3.) Mods. 

Not public. 

(EDIT: Yes, I think that letting the player know why they were barred is an [i][b]absolute must[/b][/i]. And the reason why the commentbox is post-sized, not 140 characters Twitter-style is exactly so it wouldn't be out of context, but you could quote/refer to some of the places where the player was being a disturbance, cite instances of powerplay/godmod and refusals to edit posts to remove such, state when the player last IC-posted and how long others have been waiting on them, [i]et cetera, et cetera.[/i])