A lot of Valve games (e.g. Team Fortress 2) give an error:
Could not find required OpenGL entry point 'glColorMaskIndexedEXT'! Either your video card is unsupported, or your OpenGL driver needs to be updated.
I'm using r300g with Radeon Xpress 1250 (RS600).

Do you know any good way to mock the supported extension?

I'm sure they require that extension for a reason, but I'd still be interested to try (maybe it's not used with some low settings).

I can't find much information about that extension even existing (to see it with glxinfo) http://feedback.wildfiregames.com/report/opengl/, it probably has some different name.