How to make 20% time: Just do it. Don't tell anyone. Tell that asshole trying to control your work to fuck off. When you've got something, tell the right people about it. Watch the other assholes at your job trip over themselves as they trash your work that you already got buy in from from the right people. Publish it. Launch it. Put it on your resume. You'll probably get fired because the assholes will resent that y…
So, if I'm understanding you correctly, you think the way to advance your career is to spend 20% of your time working on unsanctioned side projects in work time, writing code that will be owned by your employer that you can't publish legally without permission, all the while taking longer to do your assigned tasks and looking like you slack off for a day a week? And you think that if that gets you fired you'll move u…
Or you need to invest in automating some of your grunt work, so you can do the same work in less time. Laziness is a virtue.