Just woke up with this thought
If dark energy is thermal Hawking radiation (noting that thermal photons for one observer are virtual zero point photons for another - unitarily non-equivalent vacua) then what about the cosmological frequency shifts?
However, the fact is that the observed dark energy density does empirically fit
dark energy density = (entropy/area of future horizon)^-1
(h = G = c = kB = 1)
and this area is asymptotically constant