Could you use it to produce lye as an ingredient for soap?
I would say so, but there is so little ash I wouldn’t think it would be enough.
It takes a lot of ash and what you get is KOH which makes a very soft soap.
What you want is NaOH which makes a nice hard soap. You also want a nice pure product that you can actually weigh to get the mixture of lye and fat right, otherwise you will end up with a soap that will take your skin off. Your best bet is buying it and storing it ahead of time.