It wouldn't be a big deal to write, but I think it would be a bad idea.
Why don't you set the respective lines forehanded? You can as well set the lines according to your list "now". The check will be done by the server instead of your client, and he will be faster :-)
If you don't want to do this because the list is excessively long, mIRC wouldn't be the best app for parsing the list anyway, in text format it's worse (slow), and with multiple text files even worse... many connects and long text files would make the client lag like hell. And the "speed" advantage of a (g)Zline is gone anyway if you check the usermasks remotely.