You could try sprouting it on old stale bread, that's got a nice loamy texture and would retain "soil" moisture. The problem you run into is that other things would find this a nice substrate to grow on, and you don't want to have Penicillium mold growing along with your teff. If your "soil" has lots of nutrients, which stale bread does, then you've just fired the starting pistol for every airborne spore to try and colonize it. On the other hand, if you go to sawdust -- high cellulose material, retains water, has pore space for roots to grow, low in nutrients -- can you eat the sawdust along with the teff sprouts? I suppose so, during the siege of Leningrad, sawdust was used as filler to deal with the shortage of flour. It has next to no calories, but it does add a lot of insoluble fiber to the diet.
Maybe those rice cakes, the ones that look like disks of styrofoam, would make a reasonable medium.
Yeah, ten days really cuts down the list....and if it does last ten days watered in plastic, do you want to eat it?
I get a product called "dandi blend" for tea and coffe replacement, it is just dandelion root roasted and ground, this might work as it is root matter.
I feel like anything that supports the life of your plants and the client is going to support the bad things.......even though I gate to say this you may need to look into food grade minerals or charcoal particles, something along those lines that are less likely to begin a breakdown process.
You are right, I forget that not everyone likes that........so dry crush a carrot or parsnip? Whatever you use it might be good to boil it to sterilize before you start, that might buy a couple of days....
As for food grade minerals, sand does work in the growing part. Getting the sand off in the end was a total disaster. Tried with perlite, same problem. The roots do a good job of sticking to things.
What about just a solid rock?
Daisyblend. Tasty stuff, I had it once. Rosted barley is easy to come by and may be less bitter.
8 spoons of gelatin
3 spoons of vinegar
1 spoon lemon or mint flavor
heat stir violà
You have something sticky that is edible and may (?) help the seeds stick to a stone/terracotta/whatever surface.
Raising the vinegar portion and making it less sticky might actually provide a gel-like substance that is edible and can germinate seeds without decomposing.