What do they say? Where has it even been intimated that Solas might be such?
Nowhere, it's just my speculation, all we know about Solas is that he's allegedly a self taught apostate that came out of the woods one day to offer his expertise to the Inquisition. Which isn't suspicious at all, of course, I mean it's not like Morrigan had any ulterior motives when she joined the Warden, right?
There are hints that Felassan may be one (Not being a Dalish yet knowing more about Arlathan than them) so I'm inclined to believe my longstanding theory of the ancient elven antagonist is becoming more likely by the minute.
EDIT: Here's the text from Arl Foreshadow hinting their involvement.
Forest Fall: Truth and Legend in the Search for Arlathan
-Survivors? Poppycock!
We know the ideas for DAI were around during Origins' development (Indeed, DAII grew out of the need for a bridging story apparently), so its something to think about.