i think you're wrong in this case. mIRC doesn't display the result of the /dns if it's prefixed with a dot. So, i think it would be fair if $show returned $false in the DNS event if it shouldn't be shown. Would make it alot easier for scripts who customize the dns output. Setting variables and stuff is pretty dirty, if you ask me.
Also, /dns [-ch] [nick|address] [id] would be cool. id = unique ID that could be used in the DNS event to check if you should process the result. Sure, could be done with variables.. but it gets so dirty