Workaround: custom visual item

public RadForm1()
{
    InitializeComponent();

    this.radPivotGrid1.GroupElementFormatting += this.RadPivotGrid1_GroupElementFormatting;
    this.radPivotFieldList1.RowLabelsControl.CreatingVisualListItem += this.RowLabelsControl_CreatingVisualListItem;
}
public class MyPivotFieldListVisualItem : PivotFieldListVisualItem
{
    public MyPivotFieldListVisualItem(FieldListViewModel viewModel)
        : base(viewModel)
    {
    }

    protected override void CreateChildElements()
    {
        base.CreateChildElements();

        ZoomMenuItem zoomMenuItem = typeof(PivotFieldListVisualItem).GetField("doubleStepMenuItem", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic).GetValue(this) as ZoomMenuItem;
        if (zoomMenuItem != null)
        {
            zoomMenuItem.Editor.MinValue = 0.1m;
            zoomMenuItem.Editor.MaxValue = decimal.MaxValue;
            zoomMenuItem.Editor.Step = .1m;
        }
    }
}