Mod2X blending IS supported in OpenGL

glBlendFunc GL_DST_COLOR,GL_SRC_COLOR

of course it is. what do you think quake 2 was using? afaik quake 3 is mod 4x though for a greater dynamic range.

No, that is texture combiners you are thinking of. I am talking about polygon blend modes, not texture blends.

In what situations would this be used?

Bullet marks and decals.

Ah, I see, handy! :)