What would be a good way of inter-stellar communication for chaos? we're more or less having a regular war with a non-canon Xenos race infestating the Screaming Vortex and we need to spread our forces thin to be able to deal with them.

A Sorcery-based equivalent. Possibly a ritual, but IIRC, Magnus used sorcery to send a real-time astral projection of himself to Terra. Admittedly, you probably won't be able to pull that off, but something similar.

IIRC there's a sidebar in Deathwatch somewhere about Librarians being able to send astrotelepathic-style messages.

Or somehow acquire and reverse engineer Necron communications technology.

Sorcery/ritual is probably going to be easier.

Really, they'd probably just use psykers the same way, except that they'd subjugate themselves to the terrors of the warp while doing so, unless they would be similarly soulbound to a powerful entity (which is entirely possible).

One alternative is to "burn" psyker slaves as they use a choir to send messages through the warp and become increasingly corrupted or demented, replacing them as necessary and possible.

Another would be to use actual, corrupted astropaths, although this is probably the one that is the hardest to come by, but also the most useful, since they have knowledge of imperial communications.

Sorcery is yet another option, forming rituals to send messages through the warp, forcing daemons to either serve you or simply hold them at bay. This could involve ritual sacrifices.

And yet another would be more or less the last one, but through a ritual, strike a bargain with daemons to allow you to perform the duties of an astropath through long-range astrotelepathy, where the powerful daemon or it's servants would hold other influences of the warp at bay. This could involve a lot of potential costs, though.

Woe to any that have been pissing off daemons that try to send a message through the warp.

"Why did you transmit fleet movement coordinates to Mr N.Urgles Takeaway again?"

"I didn't, but he does make a great pizza if you can endure the toppings."

Warlord Zann discovering the breach in communications.

In all seriousness though, rogue psykers to burn as fuel to transmit, powerful sorcerers and corrupted astropaths are good. Alternative option, could consider daemonic or Xenos tech since they would not be bound to the same restrictive nature as the Imperium over what they can use.

Telepathy discipline already exists in BC, with all-too familiar Thought Sending and the rest of it - except astrotelepathy. Well, authors probably thought you won't be even needing that, but if you do - just grab astrotelepathy rules from another existing game, it makes sense.

In the HH novels, Horus linked two psykers and sent one to the blood angels and he basically used them like a telephone. It was unpleasant for the psykers but hey, thats their problem.

Sorcery allows Chaos to do so many wonderful things. A good sorcerer, mortal, CSM, or otherwise, can perform most of the functions of both Astropaths AND Navigators, albeit, in a different way of course. It is psykers and sorcerers who keep Chaos glued together and cohesive, and without them, Chaos would simply have to drift through space unguided and without much communication with each other, like all those Khorne-worshiping schmucks do already...

As far as the mechanics go, if you have access to Rogue Trader, you can simply duplicate the rules for Astropathic Communication on page 162, and substitute Intelligence in place of Willpower (as per the RT rules on Sorcery in one of the later splat/campaign books) if anyone actually wants to perform it, or you can just keep it off camera.

Of course, a rogue psyker can pick up the abilities too, but the beauty about Sorcery is, unless they are beholden to Khorne, pretty much anyone can learn it and pick it up (provided they're willing to risk their very soul being devoured as they try to learn). This means that, be you player or GM, you don't have to create a psychic character if you don't already have one to use Sorcery.

I'm assuming sorcery can solve this problem.

IIRC there's a sidebar in Deathwatch somewhere about Librarians being able to send astrotelepathic-style messages.

I'm assuming sorcery can solve this problem.

IIRC there's a sidebar in Deathwatch somewhere about Librarians being able to send astrotelepathic-style messages.

DW Librarians can buy Astrotelepathy for 500 XPs.

That sounds like a quick, workable solution.