The problem with making a solution for "most networks" is that every network uses different methods and messages when sending information to users who are disconnected (klined/glined/etc). Not all networks even disconnect users for having an invalid userid. Some networks just modify the userid to something that is acceptable (ie, changing abc/123 to abc_123).
The "ban" reason is a completely arbitrary string of words chosen by the person who set it, so without some sort of AI, mIRC couldn't know what the string of words meant. If the entire IRC community can get together and decide to use either identical ban reasons, or extend the raw system as I described in my first post. Raw codes are meant so that the IRC client can understand what a message means even if the "human readable" portion of the message differs from one server to another.
If someone is looking for more detailed d/c messages for a specific server/set of servers, then it would be simple to make a script that recognizes certain d/c messages and displays an informational dialog. Such a script would have to be constantly updated whenever the ban reasons changed, or it would no longer work for those messages.
I think a more-to-the-point solution would be for each server to provide a link to an informational website within its ban reasons. Ex, Klined- Invalid UserID, more info:
http://www.server.com/klined.html#invalid_userid-genius_at_work