Hmm, wouldn't an $iscmd be the equivalent of a $islias?
If you don't set your aliases to be local ones, then a $isalias($right($1,$len($1))) should do.
Of course, there's a catch with built in aliases, but that just makes $cmdprefix even more useful.
You set $cmdprefix to the first character and check it in the if against w/e you want.
Personally, I never use mirc's ini file for this, I usually use a hash value, which is more easily messed with, and independent of mIRC's version. Compatibility.
Then again...I guess I'm a little more advanced then your average scripter, and enjoy wasting my time on occasion, or just going for the more complicated thing for the heck of it x>
Considering you're going to use w/e will be added in an if statement, the same way you would use the existing method, I'm gonna go ahead and say this is pointless. :>