The fatal flaw both in a "chief culture officer" and a "chief dissent officer" is the assumption that this is one person's job, who sits on high, vs. being _everyone's_ job.
A job title with "culture" in it doth not a culture make.
There is of course merit in everyone doing critical thinking, certainly. I don't think one obviates the other.
I do think having someone who is culturally allowed to put on the breaks and/or seek clarification about potential bad ideas is also worthwhile, for a couple of reasons I can see right off the bat:
1. It may reduce the 'bystander effect'[1].
2. It may reduce the negative emotional impact of naysaying a popular idea from inside the group that delivers it (subconscious social pressure impact).
The role could of course be abused and become yet another abusive fiefdom, but I think that just makes choosing the right person for the role more important.
Personally, I would not create positions with those titles, but:
By creating the positions, you are sending a message to your employees that dissent is OK, or that thinking about culture is important. Further, you have an executive level person whose job it is to look for and remove barriers to dissent within your company, or to institute programs to make sure your employees are thinking about culture.
A job title with "culture" in it doth not a culture make.