I'm pretty sure it's public knowledge at this point, especially when posting about Demandred theories but RJ has said in interviews that:

1) He is not Mazrim Taim (he actually repeated himself multiple times on this point)
Taim is not Demandred in disguise.

How do we know? Because RJ said so.

Q: "It's been said that you mentioned that Mazrim Taim is not Demandred. There seems to be some confusion on whether or not you said that."
A: "Mazrim Taim is not Demandred."

[Matthew Julius, post-COT signing, Dayton, OH, January 16, 2003]
Tim Kington concurs:

Q: "Taim is clearly not Demandred, right?"
A: (Disgusted) "I've said that before, and it's not Taim, it's Ta-eeem."

The same response was also reported on wotmania.com (*awww the memories, in LiveJournal's Wheel of Time community, and in several emails.


2) As of Book 9, we have not met his alter-ego or cover identity if he has one on-screen.

All of these can be debunked or contested for various reasons, but it's not necessary to do so:

Q: "Have we yet seen the alter ego Demandred presents to the Third Agers on-screen?"
A: "No." (I asked twice to make sure.)

[Michael Martin, Dayton OH signing]
So, if Demandred is disguising himself as someone, it's not someone we've met so far.