The Halo example isn't really comparable - yes it was a new IP on a new platform and it did a great job of getting it's platform sold but the difference is that the XBOX was at least in people's mind as a gaming platform even if they weren't inclined to buy it before. Linux is not viewed as that and I can't see something like Steam OS changing that without a massive marketing push - which often means a big exclusive to showcase it and that brings me to the next problem which is that Steam is already spread across various platforms and I just can't see someone, even Valve developing a AAA exclusive game for it. Games of that calibre are just too damn expensive to develop to risk it sinking without a trace because of the platform. It's a different story for someone like Microsoft or Sony to entice exclusives because they are massive corporations with huge amounts of money to throw at it, even so it seems to be going the way that "true" exclusives are getting rarer and I might be wrong but it seems to me that many of those tend to either be developed by a studio they own or a smaller house that they take under their wing.
An existing, hugely popular IP would maybe have a good shot at raising the profile of the platform but why take such a risk with something that would be massively profitable if published in it's normal channels? A timed exclusive is unlikely to generate too much interest since people will just wait unless the period is on the long side and then the potential customers will just get resentful instead.