Re: What is the most absorbent "local" natural fiber?
I really don't know for sure, but if you can't get your hands on hemp or bamboo and/or don't have time to prep it, this is what I would try:
1. If you think he could handle microfiber as long as it doesn't touch his skin, I would sandwich it between flannel and try that. You can get microfiber towels in the kitchen or automotive areas of Walmart, Target, Meijer, etc. if your fabric stores don't have it.
2. If you think even sandwiched, microfiber would be a problem, I'd try baby loop terry. Do you have any really soft, pliable baby towels you could cut up and layer and top with some soft flannel? Regular toweling would work too, but is stiffer and might also give you that brick butt you are talking about. Do you have a TJMaxx, Home Goods, or Marshalls near you? You might even be able to find a "luxury" bamboo hand towel there you could cut up in a pinch
3. I would also try making a soaker or two out of felted and lanolized 100% wool to add to the fleece stash you made. Raid your closet (or thrift store) for an old really soft lambswool, merino, or cashmere sweater to cut up. My experience has been that if the diaper gets really soaked, wool keeps the sheets/PJs dryer than fleece. If you don't have lanolin soap, you can just dissolve some of the lanolin for nursing mothers in warm water and lanolize that way.
Good luck. Let us know what you did and how it went.