What you have to do is to use the AppleScript "call method" routine to call a Obj-C method that you create to do the work for you. You have to be a little careful about what you return to the method, as AppleScript can be particular about what it will hand across the AppleScript bridge, but a little error checking usually does the job.
Oh... and if you can get away with it, make the method a static function rather than an instance function, so you can call it with the "call method of class" version.