You're right: most of the cards that do the menu bar translucence support OpenGL 2.0. There is, however, one notable exception:
http://developer.apple.com/graphicsimaging/opengl/capabilities/
The Intel GMA 950 does NOT support OpenGL 2.0, and no amount of software tweaking will make it so. Its OpenGL capabilities, actually, are horrible. It, however, displays the translucent menu bar.
I do not think that Quartz GL requirements are the same as OpenGL 2.0. Quartz GL requires a programmable graphics card, support for pixel/vertex 2.0 shaders, and support for ARB fragment program. The GFX 5200 meets all those requirements, and again, there is documentation that the card supports Quartz
2D Extreme, which is not Quartz Extreme, but the "old" name for Quartz GL. Why isn't it supported in Leopard then?