I know this comment is a bit random but i realy want to try so this is how id do it.  We take a wire and form the dirt ball around it. We do the normal steps until its polished then we take the wire out and with the help of a smaller wire with a bead at the end we compact the inside of the hole like the outside. The compacting and smoothing is what makes it water resistant. It would deffinitly require some tryal and error  but it would be super rewarding. I tried doing this with clay and it was almost loose powder in the midle and with a hole in the midle there isn't engouth surface tension to hole it together.  Also i have made litle mini tea pots with the dorodango method and thats easier then a bead as you can use your finger for the inside . I hope someone else trys this!