Things like that are homages, references, or in-jokes, and a reference to a town called "Corwin" is not a connection any more than dropping a line from "They Live" in the movie theater in Proven Guilty was. It's more than a long shot. It's a legal problem that Jim cannot (and wouldn't want to) overcome.
Like I said, you can draw tons of parallels between Amber and the Dresden Files—you might even be able to use those parallels to predict future events—but there isn't a crossover. There legally can't be.
Maybe something like what you're suggesting happened, and God in the Dresden Files is just the guy who made this universe. But he won't be Corwin, or any character from the Amber Chronicles, the Marvel or DC Universes, or Star Trek. He or she (let's face it, with the DF and Harry's luck, God's probably a female, if she has a gender at all) will be an original creation made by Jim for the Dresden Files.
Assuming Jim can and has the desire to overcome the intellectual property issues surrounding this, if you're right, it wouldn't make sense. Even importing a character Jim created himself in AmberMUSH wouldn't fit with the Dresden Files's existing canon. It would be foolish to even try to do so, because it would require far, far more effort to justify than to make something new, and offers absolutely nothing of value to the vast majority of his readers, who have no idea what AmberMUSH is. Cameos are supposed to make a significant cross-section of your fanbase squeal in delight, like if Jayne Cobb showed up in Dollhouse. And honestly, making a character from another series altogether God would just confuse or infuriate the vast, overwhelming, near-total majority of people reading the book. Are we to be expected to read an entirely different book series by a different author with a different publisher in order to understand the backstory for a critically important character?
Please don't take this as a personal attack; it's not. I just think that theorizing like this isn't worth it.
On the other hand, I invite you to write fanfiction about this to your heart's content; I'll read it.