I think it has something to do with MSAA in dx9 and I'm afraid it's not possible.

I don't know very much about it but enabling MSAA even bypasses external postprocessing by programs like sweetfx for example. (I'm using this one to add custom pp shaders to other games :3)

Supersampling works fine, as long as the user has a PC that's beefy enough for this task.
Also, afaik MSAA is pretty much the same as SSAA - it just does some polygon edge detection and only supersamples these edges.
That's why it doesn't smooth off alpha tested stuff (vegetation, for example) and also doesn't improve texture quality, aswell as aliasing caused by things like specular highlights, etc.