I think pieces of a worn out T shirt would be fine to clean a bore. I also think the reason they used tow back in the day was not because it was a superior material, but because it was available and cheap, not being spun into thread and woven.
Hemp isn't all that water absorbent, IIRC. I want something that will absorb the cleaning solution and slop it around down there. I don't know what baler twine is made of. So about any old rag will do.
Someone mentioned nettles, and they furnish very strong cordage, but are hard to harvest. They sting, at least the ones around here do. Flax and nettles have to be "retted", when the outside sheath dries and can be separated from the fibers.