Im not sure if this is common knowledge or not so I apologize if it is. This info is not on the libretro wiki page so I am just writing it now.
Scummvm Libretro Core. (Windows Guide [Should Translate to Linux])
Running a scummvm game from command line you actually don't ever have to open the scummvm GUI. All you need is to make sure of is that the game is extracted, and has an entry in the scummvm.ini file like this:
[dig]
description=The Dig
path=C:\Emulation\ScummVMGames
gameid=dig
guioptions=sndNoMIDI
Then you need to make a small text file named after the short hand name for the game. This file only has to contain a single line of text that is also the shorthand name of the game.
Examples would be: "C:\Emulation\ScummVMGames\dig\dig.rom" inside of dig.rom is just the word "dig".
Then you would call the game direct from command line: retroarch -L "C:\Emulation\Emulators\RetroArch\cores\scummvm_libretro.dll" "C:\Emulation\ScummVMGames\dig\dig.rom"
This gets much easier if you have a pre seeded scummvm.ini file like this one and just edit the paths:
http://pastebin.com/CbmvqqYR