Actually # has historically been an allowed channel name on most IRCds, although some networks will choose to block it from use. For example, channel # exists on EFnet.

The original IRC specification states:

Channels names are strings (beginning with a '&' or '#' character) of length up to 200 characters. Apart from the the requirement that the first character being either '&' or '#'; the only restriction on a channel name is that it may not contain any spaces (' '), a control G (^G or ASCII 7), or a comma (',' which is used as a list item separator by the protocol).

So, basically, # is a legal channel name by this definition. You would have to be very careful when creating mIRC scripts to be used in that channel, although I believe it's nothing that can't be worked around.