macOS sierra 10.12 screen captures - how to edit system files?
There is no need to disable SIP and hack /System files to change the filename and location of screenshots. Use the following Terminal commands to set your preferences:

#SCREEN CAPTURE
# Show/ Don't show date in Screenshot filename
defaults write com.apple.screencapture include-date -bool false

# Save screenshots in PNG format (other options: BMP, GIF, JPG, PDF, TIFF)
defaults write com.apple.screencapture type -string "png"

# Save Screenshots to location:
defaults write com.apple.screencapture location ~/Desktop/Screenshots

# Disable Screen capture's shadow
defaults write com.apple.screencapture disable-shadow -bool true

# Change the filename
defaults write com.apple.screencapture name [yourFilename]


You may need to quit and relaunch the SystemUIServer, (or restart).

If you need a more specific workflow, then you can modify or process the files with a Folder Action script, to run when they are created.