Jerry..
... around 16 y ago (16y, sixth part of a century, god!) I've had a similar question: the saturation of some laminates (PI vs. FR-4) and applied IPC-ML950B, §2.3.20. May the Schwartz can use a similar procedure for a coated, inert substrate:

• cleaning with acetone (for conformal coating I wouldn't)
• conditioning at 105...110°C at 0%rH for 1 hour
• cooling at excincator to ambient temp
• 1st weigt
• conditioning in boiling water for 2h
• cooling in cold water to ambient temp (for 15 min)
• remove all water at the surface with a cloth, and immeadetly
• 2nd weight
• conditioning at 120°C in vacuum for 4h and
• 3nd weight...

... and turn on the calculator
However, I think the meaning of a moisture absorbtion figure is limited. More significante are electromigration tests of realistic combinations (pcb-solder resist-flux/soldering-cleaning-conformal coating)  and  a cross-cut test after clima exposition.

Or a very nice quick test to check the adhereance (indicative or comparative): exposition in a autoclave at 125°C (equals 1.31bar damp overpressure) for 30 minutes.

Bernhard