In White Night, the Ghouls say they come from "the Realm of Shadows" which Harry queries Lash about. She explains it is a region of the Never never.
This also appears to be the same place as where Cowl is operating from, and where Peabody attempts to flee. This may also be the location of the Fomor Empress (based on the underground, underwater-like features such as a mildew smell).
The ONLY connection I have found is from the Call of Cthulu rpg. Spoilers might follow here so be warned!!
Ghouls seem to be the servants of "the Charnel God" Mordiggian (a Great Old One), who also has human followers. The ultimate goal of ghouls is to corrupt humanity to the point where society breaks down and cannibalism is everywhere. They wish to return the physical realm to their god (who is currently trapped in "the Dreamlands"). His city their is called Zul-Bha-Sair "the Charnel House". There are "lesser" and "greater" Ghouls in the rpg. Which matches with the Dresdenverse (in White Knight there are those uber-ghouls). The "lesser" ghouls often have claws and fangs, but often can pass for humans and live a long time. The "greater" ghouls however are almost immortal, similar to the Deep Ones but are generally more sophisticated. The Fomor connection strikes again. In the Dreamlands, Ghouls are the ruling class over the humans. There is also a cross-breed "the Hu-ghoul" which is often weak and stupid, but can be super strong and monstrous. This is the ghoul I think that appeared in the Raith Deeps (which fits if the Fomor were helping breed them, as they do).
Curiously, this rpg also mentions Montague Summers (an author that Jim read for research when creating his Vampires). In Montague Summer's book The Vampire, His Kith and Kin he seems to say that Ghouls are just another type of Vampire. Perhaps they are the the Dresdenverse too...
What's further interesting is that the Cult of Mordiggian or the Charnel Cult started out as a ghoul and necromancer cult. So again we see this connection of Ghouls and Necromancers.
It is clear that should Mordiggian enter the physical world and be able to remain, it wouldn't be as cataclysmic as say Cthulu waking up. He tends to work slower, and would spread his influence from his emergence point. He also mostly is only interested in eating corpses.
There is also an odd South American connection, the temple that they operate out of is in South America and resembles a Zigguarat, which in term is meant to be a copy of Mordiggian's temple in the Dreamlands.
To be clear, I think the Dreamlands are the Realm of Shadows but I also think that the concept in the rpg was that of a demimonde (a hidden world).
Finally, there is this strange connection thing of the Cult's High Council. It is made up of three members, a ghoul and a human and a Hu-ghoul. The Ghoul is an unassuming man, the human is an old and powerful sorcerer and the Hu-Ghoul is a scion (Ghoul father, human mother) who is an incredibly powerful Necromancer. I have no idea if this will tie in or not to the Dresden Files, but all three of those archetypes exist so far (well, the Hu-Ghoul doesn't but could be something else..)
I think this Realm of Shadows the Ghoul's mentioned is extremely significant and I believe Jim has read if not played this rpg, so I think the connections are relevant.
The ghoul's are a mystery but I think also significant to the Black Council somehow, and I hope we will get more insight about it all in Peace Talks and Battle Ground.