There doesn't seem to be a trivial solution, so I've went with using 2 copies of Notepad++:
- (tabbed copy) Notepad++ 6.3 installation for opening files in tabs
- (untabbed copy) Notepad++ 6.3 compiled from source with a couple of hacky tweaks for opening files in new windows (untabbed).
Without the tweaks to the untabbed copy of Notepad++, files won't always open in the copy of Notepad++ that you would expect. I'm sorry if editing source files isn't suitable for superuser, but I hope this will be useful for somebody.
Compiling the untabbed copy
The source code readme recommends compiling in Visual Studio 2005, but it compiled fine for me in Visual C++ 2010 Express (free from Microsoft - http://www.microsoft.com/visualstudio/eng/downloads#d-2010-express).
Simply open npp.6.3.src\PowerEditor\visual.net\notepadPlus.vcproj (and convert the project if it asks you) and choose to build "Unicode Release". You can now test that it builds correctly:
This will place notepad++.exe in PowerEditor\bin, but you will need a copy of SciLexer.dll in the same directory before it will run. This comes with Notepad++, so simply download it from the site. Also create an empty text file "asNotepad.xml" in the same directory, which will make it untabbed and open files in new windows.
In winmain.cpp, on line 214, change
"nppInstance2", or similar. With this changed, the tabbed copy of Notepad++ will not think that a Notepad++ window is already open when you only have untabbed windows open.
In Notepad_plus_Window.cpp, on line 32, change
"Notepad++2" or similar. This is used in winmain.cpp on line 287. With this changed, the tabbed version will not include the untabbed windows when it is searching for currently open windows to open the file in.
Now build again, and move the PowerEditor\bin folder to wherever you like. If you wish the untabbed copy to open when you open certain file extensions (such as .txt), in Windows 7 I found that I had to rename "notepad++.exe" to "notepad++2.exe", otherwise it would open the installed (tabbed) copy.
If you compiled from source and want the plugins menu, you also need to copy DLLs into the plugins folder from another copy of Notepad++.