Does this need to be an absolute URL (i.e. with hostname)?  At the moment, I think it generates a relative URL.  The Course URL is being passed in because it is an absolute URL, and it's not clear to me whether or not it has to be calculated on the LMS side.  If it's just a constant that's being prepended to the front of the relative URL (e.g. classes.stanford.edu or edx.org or whatever), then calculating it on the worker side is fine.  But it should be consistent.