Actually gentlemen on the newer msn A320s you can start the APU below FL250 after both engines fail. The relevant QRH checklist (ENG DUAL FAILURE - fuel remaining) has been modified. This is possible since there will be no need to fly on the batteries only after the landing gear is extended, as the rat / emergency generator powers the aircraft until touchdown.
So once the APU is online you could theoretically power the Yellow electric pump and extend the flaps. We have tried it in the simulator during the Dual engine failure exercise with forced landing.
However this is not included in the QRH checklist by Airbus and therefor would not be recommended in real life. Maybe the pump could overheat like on the A330/340. Airbus certainly has a reason not to include it.
Best regards.