Then I tried to see if something is possible. But actually r_ext_multisample sets SDL multisample configuration and it seems to be limited to 4. I searched a little to understand why, but this is not documented. And I didn't find a safe way to set multisample in another way. Then as I'm still not an OpenGL expert, I won't dive more in such stuff.
Btw did you try to force the anti-aliasing settings with your video card tools ? I know it's possible with nvidia. I tried by myself, and it seems r_ext_multisample set to 4 gives the maximum anti-aliasing... but I can't really confirm it's true as it's difficult to really know what has been set...
So, sorry I won't help more right now, but tell us if you can force the anti-aliasing settings in some way. Eventually disable r_ext_multisample when forcing it.