In the past I worked for a company that had flight crew nominate exits and there was no issue. I had no issue. It was always the caveat that if a nominated exit was not useable, it didn't get opened by the CC. I agree entirely that there are some circumstances where the FC might just have a better idea of where the dangers are outside, dangers that won't be immediately visible when checking outside conditions from a door viewing window. I would have to research the history to my present procedures, they have been in place since day one.Most likely borrowed from someone. After all how else do most airlines get the initial manual suits up and running? The matter has been discussed a few times over the years but there has been no will to change it. Personally I prefer the FC to nominate. I then trust the CC to make the correct assessment based on what they can see. Its a grey area at best.
Not sure where the covert evac signal originated, I haven't seen it appear in a discussion forum since 2009 so maybe it has passed away into the ether.