Hi,
I was going to write some stuff, then I checked your blog and sure you did write down some ideas! I am going to repeat some of them. Nice blog!
First thing first; if I am remembering correctly,
Fukuoka mentioned that seed balls were used in Nile delta AFTER the annual spring flooding. The original recipe is not meant to withstand flood conditions. Of
course you can add additive to make clay balls hold their shape such as a small percentage of cement (it makes clay balls brittle though). Will cement have an adverse effect on germination? Didn't have a much impact on radish seeds, you might want to give it a try. Or after you finish producing seed balls, you might want to coat them with a very thin layer of cement.
The other thing I will comment: why are those seed balls too small? It would be easier to mass-produce for sure, but if you want those to endure environmental challenges, bigger is always better. I use seed balls to force an unproductive hostile land (neighbors land) to give some harvests, so I bombard the place and leave it for 6-8 months. Seed balls just sit there, waiting for the time to come. My seed balls have a diameter of min 1,5 cm, usually 2,5- 3cm (a bit bigger than 1 inch) and they have additives to make them undesirable for wildlife (grape seed powder against birds, Carolina reaper powder against mammals /any chili would do/). It rains some years, smaller sized balls germinate and fail. Bigger seed balls will wait for their turn. Other additives also help with germination: adding powdered goat manure gives them an initial
boost, coconut coir helps the balls stay moist longer for better germination rates. Each additive increases the cost and it is easy to fall into over-think/design trap. But give it a go, try to add some stuff. I suspect adding char to clay will adversely affect your situation.
A bit of overthinking for your situation: How do you get your clay? Do you let it sit in water for 2-3 days before you dry it to use? Biological activity increases plasticity properties of clay. How pure is your clay resource? Silt and sand, even for small amounts will make balls crack and dissolve. How compressed is it? You don't want voids in your clay balls if it is going to withstand flood conditions.
About mulch layer, my
experience (for larger seed balls) is that mulch heavily under seed balls and lightly over them. No much layer
should be thicker than 2 inches (5cm) when the expected germination happens (such as fall rains). Sprinkler irrigation gives higher harvests but it contradicts with the initial motive to use seed balls.
Hope it helps.