[Answer]-Overriding model fields with Tastypie

In case anyone finds this, I found a solution. The easiest thing to do was just to override the obj_create method with a new one as follows:

def obj_create(self, bundle, **kwargs):
    # Get tech for this user
    site_user = SiteUser.objects.get(user=bundle.request.user)

    bundle.obj = self._meta.object_class()

    for key, value in kwargs.items():
        setattr(bundle.obj, key, value)

    # Override the tech and operator fields
    setattr(bundle.obj, 'tech', site_user)
    setattr(bundle.obj, 'operator', site_user.operator)

    self.authorized_create_detail(self.get_object_list(bundle.request), bundle)
    bundle = self.full_hydrate(bundle)
    return self.save(bundle);