Unruh particles

A photon is not an observer, and can't be because its proper time is always zero and it has no frame of reference in which it is at rest. However, Unruh radiation is black body radiation, so it does include particles with charge and mass. To get an observer from this, you can get low-probability interactions among these particles that form a Boltzmann brain. This is known as the formation of a Boltzmann brain by nucleation, and it's the scenario commonly envisioned when Boltzmann brains are discussed in the context of the distant future of our universe, which is de Sitter.

What an accelerated observer sees as a detection of a quantum of Unruh radiation, an inertial observer sees as the emission of a quantum by the detector. I don't see why this analysis would change if the inertial observer is a Boltzmann brain. The Boltzmann brain can also say, "I think, therefore I am," but according to an inertial observer the brain never existed and never had that thought.