OK; found out that the stand-alones can only open jpegs. They can save either jpegs or tiffs. For the stand-alones, if you want, you can always do save as and export the tiff before hitting the save button to return the resulting processed jpeg back to the GIMP. Anyway, below's my list and I checked that they all now work. edit (3/28/2016):
You can open any file type (PNG/JPG/TIF) if you create a shortcut to the executable itself and drag the file on top of the executable; if you do this, just make sure do use a copy (not original). Unfortunately, for the 4 complete stand-alones, Shellout can only either use JPG or TIF (use TIF but you may sometimes get dialogue errors but GIMP will receive the updated edit).
Updated the list and added the pre-sharpener preset too (overlooked it last time since I thought it was something else; it's actually pretty cool for subtle edge sharpening without adding noise).
Quote:
["DFine 2", "\"C:\\Program Files\\Google\\Nik Collection\\Dfine 2\\Dfine 2 (64-Bit)\\Dfine2.exe\"", "png"],
["Sharpener Pro 3", "\"C:\\Program Files\\Google\\Nik Collection\\Sharpener Pro 3\\Sharpener Pro 3 (64-Bit)\\SHP3OS.exe\"", "png"],
["Viveza 2", "\"C:\\Program Files\\Google\\Nik Collection\\Viveza 2\\Viveza 2 (64-Bit)\\Viveza 2.exe\"", "png"],
["Color Efex Pro 4", "\"C:\\Program Files\\Google\\Nik Collection\\Color Efex Pro 4\\Color Efex Pro 4 (64-Bit)\\Color Efex Pro 4.exe\"", "jpg"],
["Analog Efex Pro 2", "\"C:\\Program Files\\Google\\Nik Collection\\Analog Efex Pro 2\\Analog Efex Pro 2 (64-Bit)\\Analog Efex Pro 2.exe\"", "jpg"],
["HDR Efex Pro 2", "\"C:\\Program Files\\Google\\Nik Collection\\HDR Efex Pro 2\\HDR Efex Pro 2 (64-Bit)\\HDR Efex Pro 2.exe\"", "jpg"],
["Silver Efex Pro 2", "\"C:\\Program Files\\Google\\Nik Collection\\Silver Efex Pro 2\\Silver Efex Pro 2 (64-Bit)\\Silver Efex Pro 2.exe\"", "jpg"],
Quote:
["DFine 2", "\"C:\\Program Files\\Google\\Nik Collection\\Dfine 2\\Dfine 2 (64-Bit)\\Dfine2.exe\"", "png"],
["Sharpener Pro 3 pre-sharpener", "\"C:\\Program Files\\Google\\Nik Collection\\Sharpener Pro 3\\Sharpener Pro 3 (64-Bit)\\SHP3RPS.exe\"", "png"],
["Sharpener Pro 3", "\"C:\\Program Files\\Google\\Nik Collection\\Sharpener Pro 3\\Sharpener Pro 3 (64-Bit)\\SHP3OS.exe\"", "png"],
["Viveza 2", "\"C:\\Program Files\\Google\\Nik Collection\\Viveza 2\\Viveza 2 (64-Bit)\\Viveza 2.exe\"", "png"],
["Color Efex Pro 4", "\"C:\\Program Files\\Google\\Nik Collection\\Color Efex Pro 4\\Color Efex Pro 4 (64-Bit)\\Color Efex Pro 4.exe\"", "tif"],
["Analog Efex Pro 2", "\"C:\\Program Files\\Google\\Nik Collection\\Analog Efex Pro 2\\Analog Efex Pro 2 (64-Bit)\\Analog Efex Pro 2.exe\"", "tif"],
["HDR Efex Pro 2", "\"C:\\Program Files\\Google\\Nik Collection\\HDR Efex Pro 2\\HDR Efex Pro 2 (64-Bit)\\HDR Efex Pro 2.exe\"", "tif"],
["Silver Efex Pro 2", "\"C:\\Program Files\\Google\\Nik Collection\\Silver Efex Pro 2\\Silver Efex Pro 2 (64-Bit)\\Silver Efex Pro 2.exe\"", "tif"],