Actually I just found this
http://forums.omnigroup.com/showthre...clude+contexts. This does work, although adding new contexts means you have to go through the process again. An exclude rather than an include would be better.
I then set the due dates to the past to show the number on hold in red... although this also seems to be a hack rather than a proper way of doing it.
P.S. Let me know if the benefits of OF are these kinds of hacks in lieu of functionality.