A confirmation is simply unnecessary. Can't it work like, writing this somewhere that says, by creating a pull request, you agree all your code and the discussions around the pull request is now copyright free? Saves everybody time and avoid hassles like this.
The other side of this is I get very annoyed by CLAs -- there have been a number of times I want to contribute to Google and Microsoft's open source projects, but they all require CLAs which require me to get explicit permissions from my employer to contribute to those projects. It is possible, but is a slow and complicated process that nobody wants to go through at my company. So instead of creating a pull request to address the problem, I open an issue and mention how it can be addressed. Which may or may not be picked up by someone else who wants to work on this. This is just frustrating.