Sorry for the late reply, but I have managed to create a workaround for this. I created a value converter to use the XamCalculationManager like this:

public class EquationConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return typeof(EquationConverter)
                              .GetMethod("RunEquaton")
                              .MakeGenericMethod(value.GetType()).Invoke(this, new object[] { value, parameter });
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return value;
    }

    public static object RunEquaton<T>(T obj, string equation)
    {
        return (new XamCalculationManager()).CalculateFormula<T>(obj, equation).Value;
    }
}

I can now easily add a column to my XamGrid in code and pass it the equation to be used. like this:

grid.Columns.Add(new UnboundColumn() { HeaderText = headerText, Key = key, ValueConverter = new EquationConverter(), ValueConverterParameter = Equation });

Hope this helps :D