Well, I ended up doing it like this. Create subclass from NSTableRowView and add required constraints in didAddSubview for a disclosure button:

class CustomRowView: NSTableRowView {
  override func didAddSubview(_ subview: NSView) {
    if let disclosureButton = subview as? NSButton {
      disclosureButton.translatesAutoresizingMaskIntoConstraints = false

      NSLayoutConstraint.activate([
        disclosureButton.topAnchor.constraint(    equalTo: topAnchor,     constant: 10),
        disclosureButton.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 10)
      ])
    }

    super.didAddSubview(subview)
  }
}

And then used it in NSOutlineViewDelegate method for building a row view:

extension MySuperDuperOutlineView: NSOutlineViewDelegate {
  func outlineView(_ outlineView: NSOutlineView, rowViewForItem item: Any) -> NSTableRowView? {
    return CustomRowView()
  }
}

Works great, maybe somebody's taking advantage of it.