I've had fleece prefolds before. They were nice and soft! Of course, mine were 100% organic cotton. No hemp, no conventional cotton. The trick with using thicker fabrics is to make the prefold only two layers, with only 3 layers in the center, as opposed to the 6/8 layers in twill prefolds.
I have a hemp fleece prefold. It's thicker than a cotton twill one, but it seems like it would work well. I have never used it, it came with some longies and it's embellished and too pretty to use. :LOL There are lots of hemp prefolds out there as well.
Sure, the difference is you won't make it like a real prefold that gets folded several times. You'd just cut it (after pre-shrinking) to be about the size you want and either layer it with a thinner twill or flannel & serge around the edges. Some will cut a thicker fabric and stitch extra padding down the middle before serging around the edges.