HDR isn't the issue here. Although early gamebyro releases had issues with HDR since the developers couldn't decide how to resolve 16b values to viewport color space before applying AA. Sad, yes. Even the half-life 2 code base released 6 years ago figured out how to do it. But that issue aside it is up to the licensee to implement most of these features. The base libs included with gamebyro do not do much on their own and are certainly not optimized to the point where they function properly with everything enabled.

Anyway the main reason HDR and AA don't work together with gamebyro is because of compat issues between different platforms. They decided it would be either or since compat was their primary objective. That and they thought AF would be sufficient eye-candy.

But as I explained in my previous post, I suspect that's not the only issue at work here. If you do manage to enable AA poor performance will result due to lack of other optimizations as well.