Are there any users that have had good experiences with inline water sensors and could share their experiences with me? Also, how difficult was it to determine a specific lubricant's saturation/temperature relationship to convert the % saturation result to ppm?

Greg,
Welcome to the team of those who want to solve that puzzle. We were working with Hydac AS and Vaisala M&TT and the readings we found is % of saturation @ certain temperature. This is very helpfull to us because we are just looking for that, monitor humidity levels before they become in free water.
At the time to talk with customers, they want to know how many ppm´s that mean. Every oil have a different saturation curve, this means that 75% could be 150ppm for one type of oil and 800ppm for other.
We are thinking in built individual saturation curves graph for each type of oil, begining with the most common.
If you have more information about this, please contact me directly to my email.