Either I'm misunderstanding your response to the inquiry, or you missed the point of his feature request.
What they want (and I can see the benefits) is to be able to use /amsg *BUT* have it work in the reverse of /msg #chan1,#chan2,#chan3 message in that it would send a message to all the rooms EXCEPT for the rooms specified. ie, if you are in #A, #B, #C, #D and #E, and want to send a message to all except for #C, to be able to do /amsg -x #C message to send, and #C won't get it but #A, #B, #D and #E would.
Personally, I'd like an update to /msg (I'm going to post that request in a moment)...