Tom,
You make a very solid argument equating the std QM superposition of 'massless particles' with a displacement of physically real spacetime. And that implies a rigidity at the limit of gravitational bound, however tenuous by our macro world registry devices to observe.
It also implies that a Quantum of energy along with its mass equivalence is transferred across space. I do however agree with Georgi in that the conventional interpretation of 'Quantum' is ill defined. Afterall, the quantum is an empirically derived theoretical measure of the energy transferred over the duration of one full second of time. Yet we are accustomed to counting the actual number of waves in such reception apparatus as good ol' radio. So if we accept that e=hf, then a quantum is nothing more or less than the number of waves of any specified frequency per second, and it is only by experimental estimation of intensity that it can be calculated that only a very few 'quantum' have been observed. So I am pleased that Georgina is getting down into the weeds of experimental comparison towards an ontology which will call into practice an actual mathematical analysis that makes 'hypothesis' something more than conjecture.
I think we can all agree that "massless particles" continue to be a convenient patch over an experimental lack of understanding. While this does not dispute that light velocity is the limit to acceleration of any size mass, that does not preclude that size of mass may well be the limit to that velocity being achieved. A small enough mass equivalent quantity may have a proportionate upper density bound that would be less than that exhibiting a ballistic, or parabolic function response which is the criteria by which mass is measured. And as always, I continue to hold that energy density varies in direct inverse relation to existant velocity. We would not be able to directly observe that relative density, but if we can theoretically deduce it, we would still be dependent on SR for measurement in observations. best ;-) jrc