As per the QRH the drag penalty with the leading edge devices extended may make it impossible to reach an alternate field.
When you have a HYD B system failure and need to extend the flaps & LED's using the alternative method I would suggest you only need to do this once setting up for an approach to land. Thus, when going through the decision making process, you should be very wary of your fuel state. This might influence your choice of landing field; i.e. one where you will NOT need to make a diversion if fuel is low. Once you are ready for the approach you can now lower the LED's and the extra drag etc should not present a problem. If you've fuel aplenty then you can be very generous with your extra fuel flow allowance, but i'd suggest it will be less than 'gear down ferry' inflight data; and that is available in the manuals.