Quote:
"And once you've made provision for cargo/baggage, electrical and hydraulic equipment bays and landing gear stowage, there isn't a whole lot of room under the floor of most airliners smaller than the 747."
I have to disagree. Unless the OP is excluding centre-section fuel tanks from the discussion, or limiting it to business jet-sized aircraft, his/her question is perplexing. From personal experience the vast majority of medium-to-large airliners have centre tanks.
Take the example of a modest-sized one, the A320: from memory the air-conditioning bays are in front of the centre tank and the main landing-gear bays are behind, with hydraulic bays in the belly fairings, wing-fillets and on the keel member between the gear bays.