I just read a PhD thesis done a couple of years ago on using small capacitive sensors to measure moisture. They had success, though they never achieved an accuracy better than the 2% that using relative humidity and the OPI cables can do.If you use a resistance sensor, and calibrate it for the density of the grain, you can get far better moisture readings than the sensors in OPI cables.
The problem with resistive sensors is that they corrode and will wear out over time, though maybe grain would wear them (which would cause its own issues).
For me I think the OPI moisture cables are the best and easiest way to go. I managed to decode their moisture curve data that the OPI handheld reader is using, but when I feed their constants into the formula I get moisture calculations that are quite far off of the OPI calculations. Must be missing something. Maybe they've modified the curve formula. For example, OPI is using the Halsey equation for canola, but I don't know if it's the modified Halsey equation and which version. For that matter the results from the OPI reader don't seem to match any of the standard charts I've seen for EMC. Although OPI's reading matches what the moisture was reading on the 919 when I put it in the bin, so I think it's somewhat accurate, moreso than the charts I found.