Are there any serious neuroscientific theories of emotional valence?
There is no such universally accepted model.  It seems to me that this a specific instance of the hard problem: how can matter have a subjective experience at all?

Of course we know positive experience is associated with dopaminergic action, and often the mechanism has the evolutionary function of surviving and reproducing... and we can argue the same for noxious stimuli - they signal danger in the environment and lead to aversive behaviors.

But this still ignores the phenomenology step, between stimulus and behavior; the nature of the subjective experience.  Friston's approach is interesting.  I wonder if there is any relationship to be found betweem it and Tononi's model of consciousness (integrated informatiom theory).