I have done this, unfortunately not in a way that I can easily post up here (there is an extra layer of scripting stuff on top).
I used IContactControl's for the up/down buttons. Then the problem is how to find out when they are pressed so you can change the main control - for that I used ISubect.h which you can get here:
http://forum.cockos.com/showpost.php?p=400994
Maybe I'll get time to post up an example later...