I used 20% time for things that were related to my work, but out of scope for my team. For example, I wrote a DNS library in Go for work and then I open sourced it [1] and used it to rewrite the standard library DNS client [2] as a 20% project. It actually worked out really well for me. The promo committee specifically called out my DNS project when they approved my promotion and ignored the stuff that I had been doing for my team.