After looking through gorillion of lines with 'mtar' in them you make a smart move and start thinking about how and where motions can be.
METAL GEAR SOLID V THE PHANTOM PAIN SCREENSHOTS CODE
I recommend you to clone that repository and use something like grep to wade through lua code and files fast, GitHub search doesn't work on big files and has issues in general. List of files in fpk archives in chunk0 is stored in /tpp/mgo/inf/chunk0/fpks/fpk_patches.txt. I did that before and compiled list of all files in a handy repository. Since animations have *.mtar extension, the only smart move would be to unpack all MGO files and run a search for them. They are unavailable in single player and exist only in MGO. I decided to work on DLC motions from MGO. I tried to play a "FOB victory" motion (when you get into the FOB core) in a singleplayer, but to my surprise it failed to play and "mission successful" motion (wiping forehead) played flawlessly. Since there is no unpacker for *.mtar archives and there is not a single person who wants to research the format, the only way of having fun with them is reusing motions. Anyway, every object has animations and animations are somewhat compatible between each other which is pretty cool - see this for example. Up to this date there is no unpacker since no one is interested in motions :(. Animations have `*.gani` extension and don't exist outside *.mtar archives. Player animations are stored in *.mtar files which are actually archives with different motions archived inside.