(Although, as a failed vision scientist myself, I may be credibly accused of some disqualifying bias in this regard).
For what is worth, I agree that FIM not being formalised is a point against it, but I wouldn't say there's anything ad-hoc about how it applies when fitting it to e.g. schizophrenia.
For what is worth, this quote sums up how I look at it[1]:
>Friston mentions many times that free energy is “almost tautological”, and one of the neuroscientists I talked to who claimed to half-understand it said it should be viewed more as an elegant way of looking at things than as a scientific theory per se.
1. http://slatestarcodex.com/2018/03/04/god-help-us-lets-try-to...
As predictive coding is a form of free energy minimization (under Gaussian assumptions), this implicitly provides empirical evidence.
As for the request to test the idea on live neurons, "In vitro neural networks minimise variational free energy" [1]
It is noteworthy that Friston has, as of November 2018, neither (1) formalised free energy minimisation (FIM) with sufficient precision that it goes beyond a vague research heuristic, that can (and is) adapted in ad-hoc ways; nor (2) come up with sufficient empirical evidence for his claim that FIM is how human or animal brains works -- despite the recent revolution in our ability to measure live neurons, and despite having been asked (in private) by working neuro-scientist, including at his university.