I doubt it. The way Felassan was killed in the Fade doesn't seem to be like the Warden or Morrigan. Have you read that final scene yet?
Yes. He goes to the Fade knowing he will die. His boss approaches from behind him, he explains that he is not going to take the Eluvias from Briala, that there is no harm in letting her try and that she remind him of someone. Then, we're told he never heard the blow that killed him.
This could mean anything from a lighting bolt that burned through his chest, to a fireballs powerful enough to incinerate him in a second, to a sword strike that took his head off, to a blow with a mace to even having his head removed by the paw of a wolf.
From that, it could be anyone. What we do know is that his boss wanted the Eluvian. This reduces the list of suspect to either someone we don't yet know about; such as Fen'harel; to those we know have knowledge about the Eluvians. Morrigan, the Warden, Flemeth, the Formless One.
Even the dwarven Warden was sent to the Fade at least twice, thus I'm hoping it either was Morrigan or a Warden that traveled with her through the Eluvian.