The rendering path in G.R.A.W is very different from most games in that it appears to make extensive use of multiple render targets (MRTs). (This is where one draw operation can write different values to different surfaces.)

The DX9 spec doesn't allow multi-sample AA when using MRTs and our hardware requires that all of the destination surfaces either have AA or not. This means that in order to get AA in G.R.A.W. we'd need to have lots of AA surfaces and perform a ton of AA resolves. The end result would be slow and require much more texture memory. It's not 100% impossible, and I'm not giving up on the possibility, but there is no playable solution right now.