with such a command it would become even easier to trojans to do their work abusing mIRC for it's own good. giving them the ability to kill virusscanner processes etc for instance.
I would highly suggest against such a feature ( the /pid one ).
I'm also not really sure what the $pid identifier would add personally but maybe someone can explain some usecases which show more then one person actually having use for it.
I would not like to see a /pid command for above reasons,
if the $pid() feature could be realised it would be awesome!