I have contacted 15 people with experence supercharging a Northstar and I have been working on this for a long time.
As far as hardware goes, this is a easy job. The parts are already available off the shelf. So, bolting on a Supercharger is easy and could be done at anytime.
The reason you don't see (and most likely won't see) supercharge kits for XLR's are two fold. First, Cadillac refuses to let any have the codes to the computer that controlls the car. That's a big hurtler to overcome. There have been many attempts to "hack" the code without success. Therefore, there is no way to adjust the engine to tell it you have a supercharger on board. This means that the supercharger is going to cause lots of driveability problems, error codes, check engine lights, etc. Basically, it will not run. The second problem is that there are very few XLR's out there and therefore a small potential market for anyone deleveloping such a system.
Now, like everything there is a way around all of this. There is a company call Fuel Air Spark that can set you up with a new engine control computer for a Caddy which would overide the existing system. You could use that and then bolt on the supercharger and program the engine control unit yourself. YOu will get all of the benefits of supercharging HOWEVER, your car most likely will not be street legal AND good luck getting any warranty service done. NOt to mention it would help to be a very good engineer to put it all together because your not goint to get much help from anyone.