@MartinKrpan, please note that I mentioned how I got on with my first natto experiment (using lentils) in the following WF forum thread:
viewtopic.php?f=4&t=4352My post is currently the last in that thread. I'm really pleased with the results. I fermented 2 containers of "nattified" lentils. One container could be considered a failure, but the other appears to have been a success. It smelt fine - a hint of rotteness. The gooey, slimy, stringiness can be seen throughout the lentils, in fact more so now it's been in the fridge overnight. The lentils are very soft. They taste just fine.
I think there may be 2 or 3 factors that have helped make the project succeed: temperature, and the use of a probiotic powder, and the cleanliness of the container.
The successful container was nearer the cooler fermenting box (proofer) lid, and not right next to the proofer's heat source at the bottom. Also, I wonder how much the probiotic powder helped?
One other factor could be that somehow the second container, the failed container, could have got contaminated by some pollutant, and the good bacteria may not have had a chance to thrive.
In about a week's time, I am going to try this experiment again, but use different beans - probably a mixture of small beans like mung and adzuki. I might also use some wild black rice. And I might also throw in a sweet potato, for some more experimenting fun.