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Is there, in any of the novels or background, any mention of communication between realspace and ships embarked in the warp?

There's no way to communicate, hence why ships are lost in the Warp so frequently even if they simply get lost as opposed to om-nom-nom'd by Daemons

Moreover if an Astropath gets a message from the Warp, he has no way of knowing if said message is genuine or a trick by a Daemon to eventually eat his head.