Here is my solution. Ok, it uses reflection, but I don't see where is the problem since it's much simpler, and the final usage is really just one line of code, like this:

// IComInterface
void IComInterface.ComMethod(object arg)
{
    InvokeBaseMethod(this, "ComMethod", typeof(OldLibrary.BaseClass), typeof(IComInterface), arg);
}

and the utility method (reusable for any class) is this:

public static object InvokeBaseMethod(object obj, string methodName, Type baseType, Type equivalentBaseInterface, params object[] arguments)
{
    Type baseInterface = baseType.GetInterfaces().First((t) => t.GUID == equivalentBaseInterface.GUID);
    ComMemberType type = ComMemberType.Method;
    int methodSlotNumber = Marshal.GetComSlotForMethodInfo(equivalentBaseInterface.GetMethod(methodName));
    MethodInfo baseMethod = (MethodInfo)Marshal.GetMethodInfoForComSlot(baseInterface, methodSlotNumber, ref type);
    return baseMethod.Invoke(obj, arguments);
}