I know that the Ruinous Powers are "fed" by souls feeling the corresponding emotions. And I know that the Tau's human auxilia have created something in the warp with their devotion to "the Greater Good".
With this in mind, is there a warp entity that is the manifestation of humanity's devotion to the Emperor?
It really depends, how do you look at it - the Astronomican seems like an obvious answer
The Astronomican is a psychic beacon which the Navigators use to pilot the spaceships of the Imperium through the otherwise unnavigable chaos of Warp-space. As the beam generated is psychic it exists within the psychic universe of the Warp. It is the duty of the Adeptus Astronomica to maintain all aspects of the Astronomican, including training those who will power it
If you are asking "does Emperor have a presence in the Warp" - there is the "Star Child" (although it is probably non-canon):
After Horus rebelled and destroyed the Emperor's mortal frame, his body and soul could no longer remain as one; his soul melted into the warp, only a tiny core of the Emperor's humanity remaining whole, which was like a small child in a tiny reed boat adrift in the storm of the warp. Since the Emperor's soul survived, there was a possibility that his whole essence could be reborn once more. The soul of the Emperor adrift in the warp is the Star Child.
However, this at best doubtful information introduced in the very early versions of the WH40k and it hasn't been referenced since.
There are always also Imperial Living Saints, which strictly speaking are not that different than Demon Princes - humans turned into immortal (as in "able to resurrect themselves or 'possess' another human") beings taking the power from the Warp and the Emperor himself.
The Talon of Horus book describes what could be considered a manifestation of the emperor. Where the light of the Astronomican hits a warp storm such as the eye of terror it creates the firetide. A region of warp/real space that even demons are afraid of.
Not only do they get quickly burned by its walls of holy warp-fire, these are also inhabited by warp-spirits that could be described as flaming angels; just like Chaos daemons, they don't mind slaughtering and eating chaos worshipers. These "angels" are known to cleanse their territory of all life oldcron-style, no matter its allegiance, but unlike daemons, these spirits cannot be reasoned with, as they are as insane from millennia of intolerable pain. In these regions, the Astronomican can also manifest an avatar of the Emperor's will known as Imperious, an actually way nicer chap than these postal-going "angels" described earlier, taking the form of a humble pilgrim wearing a Scream mask of gold and light.
In the book this Imperius tries to prevent Abbadon from leaving the warp and following his destiny.
The thing to remember is that unlike the chaos gods the emperor is not dead, he is a “living being” so the reverence and devotion and worship that would manifest a being actually helps feed him and adds to his power, (although not as well as eating the soul of the odd psyker or million). Now as mentioned in the other answer there are some that believe if he where to die the worship of humanity would ascend him to a level of god hood and he would become the equal of the other chaos gods. But until that happens he is relegated to just being a lighthouse and also a guardian fighting to stop chaos get through to earth via the web way.
Also remember human worship and devotion of the emperor is also mixed in with those emotions that feed the other warp entities. A flash of rage against a blasphema will be happily sucked up by Khorne, a prayer to the emperor to cure a diseased child may just as easily be intercepted by grandfather nurgle, a ceremony of devotion to the emperor may be corrupted by slaneesh and there is every possibility the emperor is actually a lord of change in a wig because, well just for the lolz :).
Humanity feeds chaos and while the masses may proclaim to be devout as with all organised religions scratch the surface and the billions of majority pay just lip service, otherwise the worship of the emperor would have already seen the chaos gods lose ascendency.
