Would the gas and dust (or at least a sizable percentage of it) in the galaxy cluster be at thermal equilibrium with the background radiation?
I'm speculating here, but I think the answer is "not likely" -- gas and dust in a galaxy cluster is in the vicinity of lots of sources of energy, and thus I would expect it to not be in thermal equilibrium with the CMBR, but much hotter. Also, there are measurements within our own galaxy of tenuous interstellar gas that is at enormous high temperatures.
Perhaps the stray atoms of deep intergalactic space have reached such a reduced density by adiabatic expansion, radiated their energy away to the point where they might approach thermal equilibrium with the CMBR, but I can't think of anywhere else in the Universe where that would be true (except, of course, at the surface of last scattering.)
But I'll defer to others with a better understanding of this.