This 'value for an "S" meter' has been thrown around for years. You can't just say S9 = 50uV without some other qualification. 50uV in 50ohms or EMF, what frequency, the specified meter dynamics (like a studio VU meter for instance) bearing in mind that most "S" meters are driven from the AGC line of the receiver with differing requirements. A spectrum analyser will have a proper logarithmic amplifier to produce a linear scale - you could produce an "S" meter conversion table.
I have two RF field measuring devices - an HP3586A and a Singer NM37-57A - neither of them have "S" meter scales. Not really good enough for a foxhunt - both weigh about 30kg/66lbs.