I was able to run OpenGL via dgVoodoo by using old WickedGL wrappers for Glide..
There are a few downsides though, like being limited to 16bit palette, scaling may not exactly work properly based on a game and some textures having blue-rave-seizures (mostly seen when playing cs_havana)..
Ive mostly tested this with half life, both steam (older protocol version 47) and WON version. The game's console reports OpenGL version as 1.1. So, my assumption, is that, specifically, for older games up to quake 2, you actually don't need to implement OpenGL 2 support with "GLSL".
btw, the reason why i didn't use D3D with dgVoodoo is because it doesnt want to wrap itself. and I'm definitely not touching the "OLE dll" thing.