I would perhaps wait until rav1e 0.4, because there is a bug in the FFmpeg wrapper (of which I am the author) right now that breaks timestamps. If you wish to build with rav1e master, you can apply this to fix it: https://github.com/dwbuiten/FFmpeg/commit/ac4cf898c1960d72bc... (removing the configure check)

It hasn't made it upstream yet since I can't bump the minimum required version in FFmpeg git until rav1e tags 0.4.