Not well evidently, otherwise they wouldn't have fled to the fade to escape the Imperiums wrath in the first place =P
The elves who entered uthenera (elven words are all italicized in the novel, lol) were not escaping the Tevinter Imperium, it was seen as achieving perfection:
"If the wrist was bare of scent it meant that the dreamer had learned to draw sustenance from the Fade itself, and would never need to be fed again. Those true dreamers were placed in beds of purest white, signifying the dreamer's achievement of perfection." (pg. 317)
This plus another comment from Felassan that I can't find right now about stories making Gods rather than the other way around leads me to believe that the Elven 'Creators' may just be the most powerful of the Elven Somniari who 'ascended' to Uthenera and achieved so-called perfection. It was also heavily implied that Uthenera is permanent. When Felassan was describing what the sleepers could have done for the elves it all involves working from inside the Fade, not waking up and using their great power back on Thedas. This would make the Elven God sleepers who can act only in the Fade, and interact with their worshippers through dreams/visions, and that certainly fits with the 'misnomer' of Gods that Fantasy settings often use that doesn't actually preclude a 'real' God from existing (and certainly doesn't prove it).
Having gone through that I'd say that I believe Felassan's master is Fen'Harel himself. At the very least it is most certainly a sleeping elf that is stuck in Uthenera. When Felassan is thinking of running from his master rather than facing his (gender assumption) displeasure he thinks of not dreaming and of blocking the Fade, so that certainly supports the idea of an Elven sleeper. If I am correct (big assumption lolz) that Fen'Harel is an ancient elven sleeper then it isn't much of a stretch to link him to Felassan's master.
This is also kind of supported by Felassan's distance from not only city elves but from the Dalish as well. Fen'Harel is neither a Creator nor a Forgotton One, he is like the Norse God Loki who walks the line between 'good' god and 'bad' god, which would mean his worshipers would doubtless also walk a much more ambiguous path, which is certainly what Felassan does.
ps: Felassan is one of my favorite characters in any fantasy setting. He reminds of Gold/Rumple in Once Upon a Time.