Nope, they made weight saving accessory compartments as replacements for the 190/5300's FDD and later the Pismo's FDD/Optical bay. Those are the ones with which I'm familiar anyway, I'll bet there are more, ISTR something about one for the 1400. One for each PB type after the 1xx series maybe? I've never heard of replacing the battery with one, unless you count the Pismo's second battery.

The PowerBook 100 is supposed to be the Luggable as slimmed down for Apple by Sony, no substantial changes were made to the overall spec from what I gather. That's why I want to check what those two User Manuals have to say about running off AC Adapter without a battery in circuit.