Was able to get some hl2 (and portal) mods to work with the Linux versions. Below is the steps needed to get this to work and the mods that have been tried. If you've tried one, you can post here and I'll add it to the list if you'd like.

Mod Steps

• Mod can't have dlls (mod can't have a bin directory unless there are .so files, then it may work)
  
• hl2.exe link no longer needed in the latest Steam Beta
  
• Copy your mod into the sourcemods directory (~/.local/share/Steam/SteamApps/sourcemods or ~/Steam/SteamApps/sourcemods)
  
• gameinfo.txt needs to be changed to reflect steampipe (thanks to Hardtimes for the idea and getting Portal Prelude to work)
  
• Ensure that your gameinfo.txt file is named "gameinfo.txt", all lowercase
  
• May need to rename your save directory from SAVE to save if you've played on Windows before
  
• Some mods need the directories and filenames to be changed to lowercase. This can be done to all files by running: find ./ | sort -r | sed 's/\(.*\/\)\(.*\)/mv "\1\2" "\1\L\2"/' |sh
  
• May need to use a new english.txt resource if the mod directory name needs to be changed. I've supplied new ones that should work

If you see a Q instead of a crosshair (seen on Blue Portals)
The issue is caused by some fonts that are missing. You can try to install the crosshair font which seems to fix the issue, either with a font manager or manually. Thanks Technogeek1994.

With a font manager

• Navigate to SteamApps/common/Portal/hl2/resource
  
• If you don't have Portal installed, Navigate to SteamApps/common/Half-Life 2/hl2/resource
  
• Double-click on hl2crosshair.ttf. You should see a font manager launch
  
• Click Install Font. Now the crosshair should show up normally (if no errors)
  
• Do the same with marlett.ttf if you are seeing letters instead of checkboxes and close buttons

If the font manager install failed, or you don't have one installed (this was tested on Linux Mint, should also work on Ubuntu at least)

• In terminal, go to /usr/local/share/fonts and go into sudo (sudo -i)
  
• Create a directory called myfonts: mkdir myfonts; then go into myfonts
  
• Then copy the fonts from the hl2 directory to your new myfonts directory: cp ~/.local/share/Steam/SteamApps/common/Portal/hl2/resource/*.ttf . (make sure you include the period)
  
• Exit out of sudo: exit
  
• Run fc-cache

Known Issues (issue and mods that have it)

• None currently

Below are the gameinfo.txt for each mod (the pastebin links), and any extra steps that may need to be done.

HL2 Mods

• Mission Improbable: http://pastebin.com/cSrmBP91
  
  
• Research and Development: http://pastebin.com/1Sfpxfjr
  
  • mod directory name needs to be changed to Research and development
    
  • Copy into new file resource/Research and development_english.txt: http://pastebin.com/KBN4myqB
  
  
• Titan: XCIX: http://pastebin.com/7tEdCCK2
  
  • mod directory name needs to be changed to Survivoriz
    
  • Copy into new file called resource/Survivoriz_english.txt: http://pastebin.com/i1mfMacN
    
  • The folder Survivoriz/materials/models/weapons/V_crowbar should be changed to v_crowbar (otherwise the wrench ingame is a purple checkerboard)
  
  
• Station 51: http://pastebin.com/umK9xwTq
  
  
• Spherical Nightmares (thanks to Big Mac): http://pastebin.com/q8xqp7U4
  
  • mod directory name needs to be changed to Spherical nightmares
    
  • resource/Spherical Nightmares_english.txt needs to be renamed resource/Spherical nightmares_english.txt
  
  
• MINERVA: Metastasis (thanks to Big Mac): http://pastebin.com/7htcfkZP
  
  
• Return ( http://www.moddb.com/mods/return - thanks to Vizzy): http://pastebin.com/S1sx0Zp1
  
  
• "WHOOPSERVATORY (thanks to dubigrasu):
  
  • Download: http://magnarj.net/files/whoopservatory.zip
    
  • GameInfo: http://pastebin.com/TGBHssek
  
  
• Silent Escape (http://www.moddb.com/mods/silent-escape - thanks to dubigrasu): http://pastebin.com/nzmf0fmh
  
  • Wine or other required to get files (download is exe)
    
  • Disabling HDR and bloom should help with sky issue on "Dark Forest"
  
  
• Deep Down (http://www.moddb.com/mods/half-life-2-deep-down): Correct gameinfo included
  
  • Folders & filenames need to be changed to lowercase. Can be done by running the following command in Terminal, inside the mod directory:
    find ./ | sort -r | sed 's/\(.*\/\)\(.*\)/mv "\1\2" "\1\L\2"/' |sh
  
  
  
  
• Estranged (http://www.moddb.com/mods/estranged)

Portal Mods

• Portal Prelude (thanks to Hardtimes): http://pastebin.com/yC0h42RW
  
  • Copy into new file called portalprelude_english.txt: http://pastebin.com/Z7K023Wb
  
  
• Hetzchase Nailway: http://pastebin.com/A4NaMqeu
  
  • mod directory name needs to be changed to Hetzchase nailway
  
  
• Portal Pro (thanks to Technogeek1994): http://pastebin.com/egLN4JPS
  
  
• Blue Portals: http://pastebin.com/uraT7prY
  
  • mod directory name needs to be changed to Blue portals
  
  
• Rexaura: http://pastebin.com/8SYrTQLA
  
  
• Gamma Energy: http://pastebin.com/ygjMzaz7
  
  
• Factum Solus (Episode 1): http://pastebin.com/kRtehAHC
  
  
• Factum Solus (Episode 2 - includes both episodes): http://pastebin.com/zJUGuk3z
  
  
• Aperture Deja Vu (thanks to Psykotik): http://pastebin.com/z5aBxPt8
  
  
• Portality (thanks to Psykotik): http://pastebin.com/sNPSctbS