This is true. Thanks for the motivation. I'm hoping that I can pair with someone for the 20% time to try to launch a new project and satisfy my appetite for "starting up".
But you can have one of you want one.
A common reason people do 20% work is to test the waters of another area of the company. It's also a kind of tryout for you.
You do tell your manager in this case, yes. They're not supposed to say no. Hard to say what the range of responses is in practice. In my experience managers are supportive of this.