This post states that Hawking's last research project involved an attempt to invalidate the "no hair theorem." In light of that I thought it would be interesting (as well as a fitting tribute to Hawking) to revisit this question that I asked 6 months ago. The question did not receive an answer but a comment by @BenCrowell served to douse my curiosity. I repeat his comment below:
"The standard no-hair theorems are rigorous mathematical theorems that have been proved. However, they have certain hypotheses that must be satisfied, e.g., that the solution must be electrovac, i.e., GR with an electromagnetic field. The Dvali paper appears to be talking about GR with other classical fields. That may be of theoretical interest, but is probably of zero relevance for astrophysical black holes. Even electric charge is basically irrelevant for astrophysical black holes."
Now that Hawking has weighed in on the subject my curiosity is again aroused. His work suggests that the event horizon may preserve information on many more quantities that mass, spin, and charge and that this is necessary to save quantum mechanics. My interest in asking the question originally was associated with the possibility that black holes might retain the baryon numbers of the objects from which they are formed and the still unanswered question of the nature of the gravitational interaction between matter and antimatter (see "Does Antimatter Fall Up?"). In the admittedly unlikely event that matter and antimatter are found to repel rather than attract, and the similarly unlikely case that black holes do retain a memory of whether they were formed from matter or antimatter, the universe could be very different from what is commonly assumed (matter dominated). I have already outlined some of these differences in my answer to this question where it is suggested that these possibilities could provide explanations for the baryon asymmetery and dark energy enigmas (albeit requiring a revisit of the equivalence principle of GR).
Consider the Hawking radiation that would be emitted from a black hole with a positive baryon number in the situation that matter and antimatter repel rather that attract. Rather than a random emission of electrons and positrons (the case expected for Hawking radiation with hairless black holes), the emission of positrons would be greatly favored and very high energy positrons at that (the energy determined by the mass of the black hole). Should the black hole have formed from antimatter then the Hawking radiation would be dominated by emission of high energy electrons. The likely very high energies of these electrons and positrons could significantly enhance the experimental detection of Hawking radiation. So if black holes do have hair and the ALPHA-g experiment at CERN does show repulsion between matter and antimatter, then the detection of Hawking radiation may not be such a remote possibility after all.