requiredbuilder reads the ADD and EXCLUDE environment variables for adding or excluding dependencies. Then you can call requiredbuilder like this:
ADD="pil >= 1.0-i586-1vl60" requiredbuilder ...
If you wanna test some stuff, I have added a --add and --exclude flags to the latest sbbuilder http://drop.io/k3woxwp
BTW, requiredbuilder will pick any binary dep, and uses some heuristic to add python, perl, etc as deps of scripts or apps written in those languages, but in general, if a package has a non binary dep you have to add it manually.