First of all, I have an argument addressed to people who are moral relativists, but who nonetheless think there are real human rights. That is, on some moral questions, they give all the relativist responses (who's to say?), but when basic human rights are violated, they say that is wrong. I teach ethics classes, and I can tell you there are plenty of people like that. What they don't realize is that the position is logically inconsistent. If the rights to life, liberty, and the pursuit of happiness are absolute, then there is an objective standard of moral value according to which they are absolute. I'm betting that when you point this out to them, they will admit that they believe in objective moral values, as opposed to relativism.
Premise 1 is a little more complex, in there are obviously moral philosophies do not directly ground ethics in God which are nonentheless objective. However, these systems, as I pointed out in a previous post, make metaphysical commitments that conflict with contemporary naturalism, (Plato's forms which we knew in a past life, and recollect now, is a good example, Aristotelian entelechies would be another), and these world-views lend themselves to theistic arguments in a way that contemporary materialism does not. I do think that attempting to work out these metaphysical systems in a consistent way is going to lead one at least in the direction of theism.
Objects in a naturalistic world are not supposed to have moral properties. That is ruled out by any reasonable definition of naturalism. Particle arrangements are just not going to get you there.
So if we accept the idea of objectively binding human rights, we reduce the number of world-views which are acceptable, and enhance the probability of theism, on which the existence of these values can be easily understood.