^Well, the "Boltzmann brain" idea doesn't really hold up to scrutiny. It proposes that quantum indeterminacy could cause a sentient brain to spontaneously arise by chance in deep space if the universe is old enough (i.e. with enough chances, a really improbable thing becomes inevitable), but it doesn't address the question of what happens next -- i.e. how the brain survives, sustains itself, etc. Without an environment amenable to its existence, it'd pretty much cease to exist as soon as it spontaneously arose.
So it doesn't really work as a synonym for sci-fi "energy beings," particularly those that evolved from corporeal ancestors. It's just an abstract and kind of silly thought experiment.
The Wikipedia article that publiusr
cites doesn't mention quantum mechanics, much less quantum fluctuations or indeterminacy. Boltzmann's life predated the development of quantum mechanics as we know it. According to that article, Boltzmann made his argument from a purely classical perspective, in terms of random fluctuations in entropy.
As for whether a spontaneously generated Boltzmann brain could survive, one need only assume that a habitable environment is generated along with it. That's not a big leap, if one grants that such a brain could be generated in the first place, also since the accompanying environment wouldn't need to support the brain indefinitely.
In other words, the main problem isn't whether Boltzmann brains could survive for a period of time once generated, but whether they and their habitats could be spontaneously generated at all.
Boltzmann brains aren't necessarily energy beings. In fact, the whole point is that they could be completely corporeal. The purpose of the thought experiment is to attempt to account for a perceived problem between evolution and thermodynamics.
However, as stated in the article, at present, the evidence supports the idea that biological evolution does not violate the laws of thermodynamics. This eliminates the need to postulate something as radical as Boltzmann brains to explain our existence in the first place.
And, yes, not all of the incorporeal beings in Star Trek
were anything like Boltzmann brains. One might argue that the Companion could have qualified, but in many cases, the incorporeal entities evolved out of lower corporeal forms.