As of 079ecd7 (2014-11-19), the hook script is now integrated into mpv and will be avaiblable when compiled with LUA support. It is (currently) disabled by default, to enable it add --ytdl=yes to your command-line or ytdl=yes to your mpv.conf.
Ok, thanks. Note that the plan for 0.7 would be to release in ~2-3 weeks or so. Also, I think this would get more testing if it got merged in mpv early on, so more people could easily access it (though you may want to disable it by default for now).
Anyway, I implemented the youtube-dl version check at https://gist.github.com/ghedo/edf582ec6db7732e0694/revisions: it's really simple but seems to work fine, feel free to merge it (or not if you have different plans).
Can I report bugs for this here, or is there a specific place for that?
Here or directly to me on IRC (ChrisK3 in #mpv-player on freenode).
Merged, but disabled by default. Use --ytdl.
First we have to do a lot more field-testing. It will probably be baked in like osc.lua. ETA is "before 0.7".