Texmod works with Steam games.
Eloaric posted this at the Bioware SocialNetwork site for running MassEffect2; the technique works with other games.
Instructions (steam version):
In order to use this, you need Texmod: http://www.gwiki.fr/download/texmod.zip
-Rename MassEffect2.exe to MassEffect2Original.exe
-Rename Texmod.exe to MassEffect2.exe
-Launch the game via Steam, it should execute Texmod instead of the game.
-Click on the folder icon below "Target Application" then select your MassEffect2Original.exe.
-Now, you need to select my package, click on the folder icon just above the red cross and select my tpf file.
-Click run to load the game. Some antivirus may detect a trojan, that's a false positive well known of Texmod.
Something she figured out is that if you have a ton of the .tpf files for texmod you can go through and number them (i.e. 01-modname.tpf , 02-modname.tpf etc.) and sort them that way in the order you want them to load then just CTRL+A and select all the mods in the folder and TexMod will load them all at once in order so you don't have to spend forever loading each file.
Hope this helps,