You are correct - SAS looks into the scenery.cfg file. That actually stores only folders. I haven't looked at the new install method but I assume the bgl files end up in a real folder somewhere. Can you specify that folder or find it?
The XML install method creates a separate "XML Only" folder for each addon, and places it in a default folder location under "Documents\Prepar3d v3 Add-ons" (for P3Dv3) or "Documents\Prepar3d v4 Add-ons" for P3Dv4. THIS addon folder contains ONLY an XML file relating to the addon, with the information about where the "real" addon folder is located with all the files for the addon. For instance, my FS DreamTeam KLAS airport installed using the new installers that "comply" with LM's XML addon recommendation create this folder structure for the XML file location:
Documents\Prepar3D v4 Add-ons\FsDreamTeam KLAS (or) Documents\Prepar3D v3 Add-ons\FsDreamTeam KLAS (if installed in P3Dv3)
Inside the folder is an XML file (my example shown below), which contains the path entries for the REAL folder where the installer put the addon folder (real folder location may or may not be determined by where the developer wants to put it, or name it, or whatever...some installers don't give the end-user any choice where to put it. But it WILL be "outside" of the main simulator folder):
<SimBase.Document Type="AddOnXml" version="4,0" id="add-on">
<AddOn.Name>FsDreamTeam KLAS</AddOn.Name>
<AddOn.Description>FsDreamTeam Las Vegas McCarran International Airport scenery</AddOn.Description>
<AddOn.Component>
<Name>FsDreamTeam Las Vegas McCarran</Name>
<Category>Scenery</Category>
<Path>E:\0000 - P3D Addons\Addon Manager\FsDreamTeam\KLAS\Scenery</Path>
</AddOn.Component>
<AddOn.Component>
<Category>Texture</Category>
<Path>E:\0000 - P3D Addons\Addon Manager\FsDreamTeam\KLAS\Texture</Path>
<Type>GLOBAL</Type>
</AddOn.Component>
</SimBase.Document>
Using the above method, no entry is made in the Scenery.CFG file for the flight sim, and SAS will never know the airport exists.
I'm assuming SAS would have to be re-coded to look at BOTH the Scenery.CFG file AND the entire list of XML addon folders (to read all the XML's in them to find THOSE airports)? Yuck!