No longer current on B744, but........ T/E flaps assymetry is only available with normal (hydraulic) operation. If you are down to a minimum of 30 mins electrics available (standby power only), I would think that as the autopilot is not available, any assymetry would be detected early on in the flap deployment, through the control column, i.e. before Flaps 25 is selected.
Not a pleasant prospect, but in 40 years of operation, how many 747's have suffered a total electrics failure? Not even "Eric's glider" I believe!?