There's another way to do whatever you want at work, and that's covertly. During the times you're meant to be doing other things. Depends where your desk is I suppose, or if working remotely.

I think, though, if I was running a company, I wouldn't have hack weeks or 20% time - I'd try, for as long as possible, to give people extreme latitude to work on whatever they think might be useful/fun/interesting for the company - more like the Valve model, I guess.