I’m surprised he doesn’t reach out to them as a seller. The liquid that is produced by his charcoal slime can be excreted and sold to them. If it’s combined with the barrels the fish is stored in then the smell will go away. There’s nothing stopping the town that makes the smelly fish from adding the charcoal slime excretion to their barrels. I realize there’s a limited stock, but there’s no reason they can’t make a line of products that’s limited in sale. It’s about introducing people to the flavor right? So once people get hooked on the flavor, people might be willing to come up with ways on their own, to reduce the smell.
Exactly! After all, he discovered how to eliminate the smell to the most tolerable degree by using other ingredients, promoting 3 complimenting products at the same time (deodorizing, sesame oil, shappaya) which would be a win-win-win for the 3.. But alas he didn't even manage to think of it even if he really appreciated the hardwork put in product and peddling it.
He could have imagine himself being rejected or belittled as when he was still working in his previous life and since it can be offered not even for free service (free help), it's not a loss for him if there is an exchange like promoting his products or business for recipe or idea to improve their product by offering his neutralizing/deodorizing products...

I was thinking (from the novel until now) that Ryouma could help the Shappaya seller by buying it or asking regular orders to deliver it for him or inventing food (recipies) and giving it (or selling the recipe) to help the poor guy sold his wares but he didn't. Although he have no obligation but maybe he's biased on whom he helps with.. Like his employees' village product just because they're females? or he benefits from them? Hmm..
Too bad that Sionji is absent, he might be able to help with the fish ingredient bus alas..