It is worth noting that mIRC differentiates between identifiers that take parameters and those that don't when choosing between built-in identifiers and custom aliases.

If the built-in identifier takes parameters it will always be used when that identifier's name is used with following parentheses. However, if that same identifier's name is used without parentheses mIRC will use the custom alias instead.

alias rgb return Moo
//echo -a $rgb
--> Moo
//echo -a $rgb(255,255,255)
--> 16777215

If the built-in identifier takes no parameters then it will always be used when that identifier's name is used without following parentheses. However, if that same identifier's name is used with parentheses then mIRC will use the custom alias instead.

alias me return Moo
//echo -a $me
--> Collective
//echo -a $me()
--> Moo

Some identifiers (such as $nick) are not overridable at all as they exist in both $nick and $nick() forms.

alias nick return Moo
//echo -a $nick
--> * /echo: insufficient parameters
//echo -a $nick()
--> * Invalid parameters: $nick

Unless I have missed something in my skimming of this thread, this would mean that $noop() does not need special treatment.

-insert vaguely amusing put-down here-

As for the actual suggestion:
$noop() would be a nice addition

Given the fact that mIRC is often modified with backwards-compatability for scripts in mind, it would seem to be a break with tradition to treat /noop specially. Due to that, and the points made by previous posters, I would prefer that this was left alone.