Earlier quoted context omitted.
I don't think there's any official way to play the original 2004 version (or even the Source 2006/Episode One version either). The Xbox version is probably closest but they used palettised textures for the Xbox version - something that no PC version of Source ever supported - probably to get it to run okay.
That's such a pity, I always wanted to play HL2 on DirectX 6 mode.
I've also wanted to run HL2 in DirectX 6 as well on period correct GPUs. Specifically a TNT2 Ultra and a Voodoo 5 5500 I have laying around. I just haven't gotten around to it.