It may be possible to use Ech2o probes in this type of application but the installation may be tricky. You will definitely want to do a custom calibration for this substrate, and I am concerned about compression of the foam as you install your probe. If inserting the probe changes the density of the material (squishes it) then your reading, even with a custom calibration, will not be representative of the substrate. If you can wait a couple of months our greenhouse sensor (with stainless steel nails instead of our usual prongs) will probably work the best for this.

Making a couple of pilot holes for the sensor may help maintain the foam's density. If you can install the sensor such that it gets good contact with the material without changing its density I think it will work fine.