Doesn't your 20% time project still require managerial approval? You can't just work on whatever you want, you need to get it approved by a manager? This is what I've read, I don't know if it's true or not.
I personally think "managerial approval" is just for 3 things:
1) If someone decides to do something wrong/illegal as 20% project, the manager should've caught this (so this 'approval' is really a 'we reserve the right to blame the manager if things go wrong') ;) 2) People probably want their 20% project (and progress therewith) to be factored into their perf, as such, their managers need to know about it. 3) So that people don't pick clearly fraudulent 20% projects (such as 'my 20% project is to stay at home and watch TV')