Still need to try:
From an admin cmd:
reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\notepad.exe" /v "Debugger" /t REG_SZ /d "\\"C:\Program Files\Notepad2\notepad2.exe\\" /z" /f
The /z flag on notepad2.exe makes it ignore the next argument which is needed for this reg key replacement method. Otherwise it opens C:\Windows\System32 as a text file.
Optionally include /c to fill new notepad windows with current clipboard contents
For example: \"C:\Program Files\Notepad2\notepad2.exe\" /c /z