Will the g-factor of charged black holes ever be measured?

Question

Measuring the g-factor implies measuring the angular momentum and the magnetic moment.

Can these two quantities be measured for black holes?

Answer 1

the g-factor of a black hole

is

$\frac{1}{2\pi} \mu^{s} + \lambda^4$

Comments

Can you give a reference explaining the symbols?

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According to Straumann, "General Relativity", the g-factor of a Kerr-Newman black hole is 2. The expression given in the answer appears to be a theoretical result based on some model of a black hole. The question, however, is about measuring the g-factor. Such measurements, in particular if the result is different from 2, would be very interesting with regard to theories of gravity.