Getting back to the A330/340 fuel transfer. I haven't flown them for some time now, but back then I noticed that with fuel in the trim tank the horizontal stabilizer used to stay at around 2 degrees up with the C.G. maintained between 38-40 % MAC through forward fuel transfer, well within the normal C.G. range. So there was never a situation when you would have zero down-force on the stabilizer and thus the aircraft was always stable in the longitudinal axis. All the aft transfer would do is reduce the down-force to a certain extent and the associated induced drag. It is similar to loading an aircraft without trim tank to achieve an aft C.G., the difference is that the trim tank allows you to maintain the optimum C.G. location for a longer time as fuel is burned in flight.
If I remember right the only problem would be a failure of the forward fuel transfer. In that case the fuel in the trim tank would shift the C.G. further aft as fuel is burned in the main tanks and you would leave the normal C.G. envelope after some time, I think the time limit was 4 hours and there is a QRH checklist for that. I guess if you exceed this time yo u might end up with reduced longitudinal stability.