it doesn't really need a full change in the core, but it does require a db change, which won't happen at this time. This could be done with a module that writes to it's own db though.
On a personal note I think "You are not permitted to be on this channel" is a perfectly valid reason and all the reason a user needs. I don't see why more info is needed, the message tells you in plain english why you were removed. There doesn't need to be a reason why you are not allowed in the channel since IRC is not and never has been a medium where the IRCops/ChanOps were/are required to explain their actions. "Because I feel like it" has always been a valid reason for any removal.