Reformed teaching is not where I’d normally look for a defence of Christian pacifism. More often I tip toe through the TULIPs and listen for the voices of the more radically reformed.
And yet, surprise, surprise, it would appear that some at least find ample justification for peacemaking in the teachings of Calvin, without recourse to the much maligned Anabaptists. Check out this article by Seumas Macdonald on “Why solid reformed doctrines are the basis of a biblical pacifism.”
In contrast to the Christocentric approach I would take, MacDonald takes a more theocentric approach. I’m not entirely convinced that nonviolence can be reconciled with the teachings of the magisterial reformers, but I take it as a conversation starter.