What kind of signal does a databus send? If its just a 12V supply a little after ignition on, I would think that would be easy to bypass with a little switch. But I would suspect there would be a way to bypass the databus.
I had the pleasure of reconstructing a 2001 flood total. I replaced most of the submerged electronics with modules from a 2000 DTS collision total. I learned A LOT about how the network works and what needs done in order for it to all function properly. See here: http://turbojimmy.4t.com/dts/dts.html
It's digital serial data - just like a computer network. Actually, in the manual they describe the data bus system in the car as a small local area network. The radio in the 2000+ Devilles are part of that network. The major modules are the traffic cops (like the Body Control Module, Dash Control Module, Body Control Module, etc.) and other modules just provide data for them to make decisions (like the inside temperature sensor) or are "slaves" like seat heaters and the radio. Each module registers a unique code with its master. If you replace a certain module, like a radio, and its code is different than what has been registered with the master, that feature won't work. The dealer can go into the master (like the BCM) and manually register the module. I think part of the reason for this is to thwart black market parts, and the other is to ensure a revenue stream for the dealers.
It wouldn't be worth the hassle of installing a BCM in the car, because you can't just have a network consisting of the BCM and the radio. A BCM is going to want to talk to other modules and they're not going to be there unless you duplicate the entire network. Each of the "master" modules is flashed at the factory with that particular car's options. For example, a BCM in one car is going to be looking for heated seat modules, others may not. The PCM is flashed with engine management parameters associated with an individual car's drivetrain.
Anyway, on the later model Devilles, there are 2 reasons that the radio is connected to the BCM. The first I already mentioned - to register itself with the BCM. All the later model radios are going to have a version of TheftLock. There are ways to unlock the radios, though. The second reason is that the radio actually provides the "chime" functions. That's why, until fairly recently, it was a pain in the butt to put an aftermarket radio in many GM cars. But, there are modules now that talk to the BCM in those cars to simulate the chime function independent of the radio. The newer radios are also going to have satellite radio, OnStar and even navigation functionality. I don't know if they'd be upset if they couldn't find the modules associated with those features when powered up.
In my opinion, for the time, money and hassle, there are far better aftermarket alternatives that fill the double-DIN hole in the dash very nicely. The stock speakers in the Fleetwood are pretty stout so if you get something with a bit more power it sounds pretty good.