There's nothing stopping you from running a game in the Ultima Segmentum.

edit: This is a problem that has nothing to do with FFG's pet sector. They could easily just release stats for the Tau or any other xenos, regardless of where they set most of their fluff.

Also, consider setting your DH campaign in the Jericho Reach. It doesn't feature Orks or Dark Eldar extensively (for that, go to the OW setting) but it does have pretty much everything else and there is no reason why all of the resources designed around your PC space marines interacting with inquisitors and such can't be turned into your PC inquisitors interacting with space marines and such. I have already made plans to do something like this at some point in one of my DH campaigns. Newly conquered or reconquered planets are no doubt rife with heresy and recidivist and resurgent cults and the constant threat of them being followed by invasion, whether those groups operate at the behest of genestealer, Tau, or other masters. Honestly, even many of the adventures can be adapted. You cannot tell me that the more investigation oriented DW adventures make more sense for a team of space marines than they do for throne agents. Edit or don't the combat sections depending upon the rank of your warband and move on. The reality is that most DW adventures seem to be infatuated with ranks 1-3 anyway so a higher rank group of acolytes might even be able to just take them on largely unaltered.

There's nothing stopping you from running a game in the Ultima Segmentum.

edit: This is a problem that has nothing to do with FFG's pet sector. They could easily just release stats for the Tau or any other xenos, regardless of where they set most of their fluff.

I probably will end up creating my own Sector for my next campaign, but a pre-made Ultima Segmentum sector would save a lot of time...

And while it's true that the 'Fluff Police' are unlikely to kick in my door and arrest me if I use Tau in the Calixis Sector, my main deterrent doing so is the fact that I play WH40KRP specifically because I'm a fan of the incredibly detailed 40Kverse; so blatantly violating the fluff kind of defeats the purpose...

Also, consider setting your DH campaign in the Jericho Reach...

I've thought about it, but the fact that the Jericho Reach is supposed to be a semi-isolated war-zone is too limiting for the kind of adventures I want to run...

If memory serves there are over 60 worlds liberated by the initial push of the Orpheus salient alone before hive fleet Dagon arrived to mess things up. That should suggest a fair number of world even not counting the fortress worlds, war worlds, etc. Seems to me there would be plenty of room for wandering off of the beaten path and finding what you want. Even so...

The Tau and Tyranids got there somehow. presumably the big bad warpstorms that conveniently cut off the sector for over 5000 years are dissipated in places. That is a large part of the strategic value of the place, after all. it is not far from the Realm of Ultramar and the trade routes through it to the Realm are valuable etc. blah blah blah.

So you could very comfortably wander outside the Reach when you wanted something a little less crusade oriented.