Well if you provide a number for that parameter then mIRC interprets that as meaning you want the nth nickname, so there's not much point fixing the 11 digit limit because of course you'd never have that many people in a channel at once.
mIRC doesn't expect a nickname to be all digits because all IRC RFCs define a nickname as having to start with a letter. While you can still find out if that nickname is a "superowner" by looping through $nick(#channel, n, '), I suggest you inform the admins of the network that breaking protocol for the sake of allowing people to use ridiculous nicknames like that is a really bad idea.
