Gerald, you have me curious with
"Ultimately it is clear that there is no real need to have a works council in Orgs operating in Holacracy."
What makes you say that? Where do you take that clarity from? (I think I can see how that might be true, but I could just as easily argue the opposite at the moment, I think.)
I have worked with a large union in the past, and resonate with what Karilen says: I'd aim at including it fully, mapping it out as it is, and then letting it evolve through tensions. This might actually be attractive to unionists, as they might see it as a way to gain _more_ influence in the organisation! (Which _then_ even I could see as a good thing!)