Further testing has shown that HKCR\Notepad++_file is created when you register the .txt extension via Notepad++ -> Settings-> Preferences... -> File Association. Here's an export of that key:
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\Notepad++_file]
@="Notepad++ Document"
[HKEY_CLASSES_ROOT\Notepad++_file\DefaultIcon]
@=""C:\Program Files\Notepad++\notepad++.exe",0"
[HKEY_CLASSES_ROOT\Notepad++_file\shell]
[HKEY_CLASSES_ROOT\Notepad++_file\shell\open]
[HKEY_CLASSES_ROOT\Notepad++_file\shell\open\command]
@=""C:\Program Files\Notepad++\notepad++.exe" "%1""
Note that exported registy strings have escaped quotes (") and escaped backslashes (\). If you create the key directly through regedit, the command value is:
"C:\Program Files\Notepad++\notepad++.exe" "%1"
Keith