Oh man, I thought I did a write-up about how to get HDTP to work within Steam.
Well, the long and short of it is, you need a copy of DeusEx.exe from a non-Steam copy of the game. Then, replace the
Steam DeusEx.exe with the
retail DeusEx.exe and it should work.
And since you're trying to use HDTP through Steam, you'll need to replace the
current HDTP.exe with a
retail DeusEx.exe renamed to HDTP.exe. The reason for this is that the HDTP installer (if I remember correctly) makes a copy of DeusEx.exe that's found in the ./System/ folder; since the DeusEx.exe in the Steam folder is broken to only work through Steam, HDTP.exe is also broken.
Does that make sense? Let me know if you have any questions

NINJA EDIT: Oh yeah, I also found out that the
Steam DeusEx.exe doesn't like OpenGL mode at all, even with the new renderer in place; it's like Eidos enforced D3D functionality when they released it on Steam. If I remember correctly, following the above steps also gets OpenGL working properly.