I have both the Twin Otter and the Bronco installed with the XML addon method. The overview is to point the installer to a dummy folder and then create the xml file manually. Below is a screencap of all the XML installs so far. Of course, for each addon, there is a corresponding folder and XML in the "My Documents" folder. Many aircraft addons also require editing of ini files for custom sounds, etc.
I've even managed to get Carenados installed this way.
Beware, there are still many addons that have relative file paths hard coded in their files that preclude from using this method.
Hope this helps, it can be done.