I also found a super cool article with moisture content specs in the Sept/Oct issue of GVH's newsletter > under the heading > drying hops.
http://www.gorstvalleyhops.com/uploads/September_newsletter.pdf
Yep, that's the only reliable way to do it that I have found. Take two samples of equal weight and volume. One you dry until all the moisture is gone. The other you put in a bag that will allow air to pass through and put it in the oast. That sample is removed periodically for weighing purposes.
The fancy way of doing that is to get a specialized chamber that includes a scale. You place the sample in and it weighs the sample. It then drives out the moisture until the sample no longer changes weight. It can then give you the dry weight and moisture content. We just picked one up and they aren't cheap.
There are various moisture meters available that look at the electrical impedance through your sample. They work really well on drywall, wood and other solid surfaces. They also work well with dense product like corn, wheat, baled hay...and baled hops. They don't work so well with loose hops because there isn't necessarily a continuous electrical connection. so therefore you don't know your moisture content until it is all baled up, which is too late.
Yes, they also come with cup samplers. Its a cup that you shove the sample in to measure. I've tried a few out. I could get from it the general direction and rate of moisture removal but not the actual moisture content.
The problem with these devices is they need to be calibrated for the product and the temperature. Using a device calibrated for corn will not work for hops. Using a device calibrated at 80F, will not work on hops at 30F or 120F.
If you are really interested, Samual Jackson
www.samjackson.com seemed to have equipment that was calibrated for baled hops. That's about the best I could find.