I think a Trent engine windmilling will keep a genny on line, providing the TAS remains reasonably high, so the partial ping will not necessarily coincide with fuel exhaustion time.
Nope, not even close. IDG and VSCF will drop off line at ~50% N3, windmill N3 is around 15-20% even at a high airspeed. At 20% N3, the FADEC will still be powered and there will be some hydraulic pressure from the pump (although not enough to control the airplane - that's why there is a RAT), but that's about it.