If you want to enumerate the available COM ports on your WM device, there is a very simple solution: look under the HKEY_LOCAL_MACHINE\Drivers\Active for all keys that have the Name value starting with the "COM" string. With some luck, you can even find a FriendlyName value under the key specified by the Key value.
The only problem with this very simple method is that it does not enumerate the Microsoft Bluetooth stack emulated COM ports. If you create a partnership with a device that has the serial port profile, you will be able to assign a free COM port to it. This COM port works like any regular serial port but does not show up in the list of active drivers on the registry. Microsoft decided to enumerate all Bluetooth virtual COM ports under HKEY_LOCAL_MACHINE\
Software\Microsoft\Bluetooth\Serial\Ports. To find all available COM ports you must enumerate all the child keys and look for the Port value under each key.
Interestingly, the Widcomm stack does put the virtual COM ports where they should be (or at least where I expected them to be).