Center external feeds the internal wings and external wing tanks. It’s not just possible, it’s mandatory. It uses the same fuel pipe from the AAR receptacle. There are various pumps in the system: the electrics are going to be independent of engine, the pressure fed ones use bleed air but I think they are regulated so probably the same regardless of bleed air pressure. Maybe at idle the bleed air is below the regulation pressure I don’t know.
Trapped fuel is based on fuselage fuel totals, the five tanks F-1, F-2, A-1, FWD RSVR, AFT RSVR, basically the sum of the two needles in NORM. Wing internal tanks aren’t included.
Trapped fuel warning has a bunch of and-type conditions:
FUEL QTY SEL knob NORM
Fuselage fuel quantity < capacity - 500 (> 30s)
Total fuel > fuselage capacity + 500 (> 30s)
FF < 18,000 pph (> 30s)
There’s no reason that I can see that trapped fuel warning even “might” (as BMS -1 says) happen with wing tank emptying. It would take fuselage fuel being 500 lbs less than fuselage capacity for a warning to even be possible.