I have made a TEST TWRP version for encrypted /data partition on Pixel C !
( Update.ZIP_TWRP_dragon_3.0.2-0_v5.1.zip )
This one is not working yet !
It was a test .. I have implemented the "original" TWRP mechanism for decryption, ported from Google Nexus 6P and 5X.
But Pixel C is using other libraries to decrypt .. "angler" and "bullhead" are using the same way of decryption.
But they are not "mixed" ChromeOS devices. 
I would like to start the discussion and ask for help and input on how to port the TWRP decryption to Pixel C.
Maybe will this will help to get an "official" TWRP version too.
In my opinion, this would be the best solution for Pixel C – to get "official" TWRP releases from core TWRP team, on regualr base.
Like for all the other official devices too.
If you like to download and extract my "beta" version .. pls go ahead and modify as much as you like.
download: https://onedrive.live.com/?authkey=%…9EB75F5DD9DA59
This was just an idea to "copy" the mechanism from "angler" and "bullhead" to "ryu".
In the next reply I will explain what I did in detail ..
Thanks for your help