All the passenger aircraft I have worked on, B727,737,F28, D8, F27, F100, SH360,Bae146,ect. The bagage compartments are covered in a fire retardent material similar to fiberglass. All the seams are covered in fire retartdent tape, and there is no real duct going to pressurize it specificaly. Picture the fusalage as a can, with a floor across half of it. Under this floor you put a box. Then you put in a tube to pressurize the top part of the can, and a valve to limit how much air stays inside. Everything in the can will be pressurized. Inside the box under the floor, (which is sealed,(not totaly air tight))is a fire retardant system consisting of a detector and a extinguishing agent. So if you get a fire in the box, some smoke may come out, but the extinguisher puts it out, and you may then get a bit of smell from the extinguisher. This is a symplified way of how bagage hold are in passenger a/c.