Quote from Mach Stall:
"In my question I was excluding center section fuel tanks contained within the wing's pass-thru structural center section."
Thanks for the clarification. Sorry I can't help on regulations.
Quote from DaveReidUK:
"The other side of the coin is that many (most?) current narrow-body airliners have ACT provision, but only at the expense of baggage/cargo capacity."
In terms of weight (mass), there is a trade-off that applies to any transport aircraft, for the usual structural reasons. In terms of volume, are you suggesting that on narrow-body airliners the centre-section void occupied by the centre tank could otherwise contain baggage/cargo?