The accepted answer was correct at the time. However the post and preprocessors work in Flask-Restless have changed. According to the documentation:
The preprocessors and postprocessors for each type of request accept different arguments, but none of them has a return value (more specifically, any returned value is ignored). Preprocessors and postprocessors modify their arguments in-place.
So now in my postprocessor I just delete any keys that I do not want. For example:
def api_post_get_many(result=None, **kw):
for key in result.keys():
if key != 'objects':
del result[key]