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.
This would basically not work at all. Checking whether the first character is the command prefix character is absolutely necessary, as it is literally the only distinction between a command and a regular line of text. Whether the command is a scripted alias, built-in mIRC command, or server command is irrelevant, so $isalias does not help.
Even if there is a convoluted workaround, it would certainly seem more ideal to simply have an identifier like $cmdprefix or $iscmd instead. (Personally, I think it's a waste of time for mIRC to even support alternate command prefixes, but that's beside the point.)