My understanding of tail (trim tank) fuel is to reduce the amount of trim and therefore drag, not to swap pitch up trim for pitch down trim.
Sure, tail fuel is used to reduce trimmed drag, but do you really understand how it does so?
Negative lift (pitch up trim) means that to balance the overall lift/weight equation the wing must produce more lift than just the aircraft weight to offset the negative tail contribution and that means more drag due to lift. If you move the CG aft so that the amount of negative tail lift is reduced then you will reduce the wing lift requirement and get a lower trimmed drag. Going further to the point where the trim is pitch down (positive tail lift) is merely an extension of that process. So swapping pitch up trim by pitch down trim is exactly what you need to do to reduce trimmed drag.
The a/c still needs to be stable. Especially air transport category, to comply with regulations. Think about failure modes.
Read again what I said a couple of posts ago:
QUOTE]Just to round things off, the complete aircraft (tail on) aerodynamic centre will be around 50~55% mac so the aircraft would be stable even at the aft CG limit (as it must be of course to meet regulations) [/QUOTE]
Not much to be doing with failure modes either - if the CG is ahead of the aerodynamic centre the aircraft will be statically stable whatever the FBW is doing.
If engines are lost with the a/c in non normal trim, hand flying, whether it be fly by wire in direct law or traditional control systems, will be very tough for an average pilot.
This has nothing to do with engine failure (except if you are fool enough to take off with full aft CG and get an engine failure near Vmca, in which case the lateral control might be a problem, but even then the aircraft has to be certificated for engine failure at aft CG). And if the aircraft is certificated for flight with an aft CG then that is
NOT non normal trim. Moreover it has to be flown without difficulty in that state by an average pilot - that is part of the airworthiness code.