I do realize that you can adjust the individual coordinates. What I am hoping for is an API accessible way to do the same thing that is accomplished via the Trim Zone dialog in the user interface -- select the zone, tell ArchiCAD whether you want to trim the top or base of the zone and then ArchiCAD adjusts all of the zone coordinates automatically.
There isn't any such function in the API. You can modify the zone and related memo data, though. The cut is accomplished through the 'gables' member of API_ElementMemo.