I missed it loooong ago.

The Tengu and Fox masks are now available for sale here on GOG:
https://www.gog.com/en/game/little_nightmares_fox_mask
https://www.gog.com/en/game/little_nightmares_tengu_mask

They don’t need to be purchased or have a code. They are built into the game. See this very subforum for how..

Could you perhaps give a link to the relevant post?

I don’t wanna give a link for GoG to go and delete the thread. I’ll just duplicate the info here
Sound track -preorder exclusive ( purchase post release)
Wallpaper - preorder exclusive

Masks

Teapot - preorder exclusive - "1352220891"
Scarecrow - preorder exclusive - "1268928435"
Tengu - bandai namco web site vip area - "1611302854"
Fox - bandai Namco web site vip area - "1828170949"

Expansion pass dlc 1/3 - out now
Expansion pass dlc 2/3 - out now
Expansion pass dlc 3/3 - out now

how to unlock without gog dlc:-

1.create a folder in the game directory
2. give it a number listed above depending on what you want
3. create a text document called "dlcinfo" inside the newly created folder.
4. the text document (txt) should contain "DLCs are entitlements."
5. dlc unlocked. this work for the pay dlc too but i'm not going to explain how to unlock them because i don't want to get in trouble from gog/publisher/dev since it's pay content

Thank you