The long awaited guide for all this goodness can be found here: http://r3pwn.github.io/sourcemodding.html (Thanks r3pwn)
HALF LIFE 2 EPISODE 1 PORT HERE: http://youtu.be/pd-t2TrDtfU
Eventually I'd like this to help developers familiar to the Source engine and I port popular HL2 and Portal mods to the Shield.
IF YOU MAKE AN IMPORTANT DISCOVERY REGARDING EITHER OF THESE GAMES ON THE SHIELD, PLEASE POST YOUR DISCOVERY AND I WILL ADD IT TOO THIS LIST AND CREDIT YOU FOR IT
These references cover the games at versions 23 (APK) & 22 (DATA)
Hopefully you already know how to get these files from your Shield to your PC, because I’m not covering that.
main.22.com.nvidia.valvesoftware.portal.obb – Core game data, contains Source engine core and Half-Life 2 data files
main.22.com.nvidia.valvesoftware.halflife2.obb – identical to ^
patch.22.com.nvidia.valvesoftware.portal.obb – Contains all the data files for Portal
patch.22.com.nvidia.valvesoftware.halflife2.obb – Contains only the maps for HL2 and associated files (BSP, PAT, LMP)
These files can be found in sdcard/Android/Obb, you might need to enable the viewing of hidden files in your file browser to get to them. Open them as if they were ZIP files.
Apparently you can't change or replace files directly in the .obb files because of a checksum/signature check, if you do the game will attempt to redownload the .obb file (Thanks nex86)
Custom files can be loaded from "/data/data/com.nvidia.valvesoftware.halflife2/files", but like all good things this will require your Shield to be rooted.
More will be posted when more is discovered.