If the hypothesis was true, we would expect the effects of brain damage to reliably create a richer and more psychedelic experience for the sufferer. Hallucinations and paranoia are only a tiny subset of the effects experienced by people on psychedelics, so logically the only part of the hypothesis that becomes more probable is that the brain usually filters out hallucinations and paranoia. But I won't accept that either, because neither brain damage or psychedelics reliably produce paranoia, and brain damage does not reliably produce hallucinations.

I don't rule anything out, but the filter hypothesis seems a lot less likely to me given the evidence, than the simpler explanation: That psychedelics just mess up the normal functioning of the brain to create a different experience. This requires less assumptions than the filter hypothesis, which is akin to suggesting that the brain is actually drunk all the time, but alcohol merely removes the filter to allow us to experience the drunk phenomenology.