Would someting like Streamkeys [ https://chrome.google.com/webstore/detail/streamkeys/ekpipjo... ] work?

I'm using Keysockey Chrome extension for YouTube (and many other websites):

https://github.com/borismus/keysocket

(I use Keysocket it every day. However, media key handling breaks when I have apps like VLC running at the same time on OS X. As far as I know, it's a Chrome bug, because it ignores SPMediaKeyTap, defacto standard library to handle media keys on OS X.)

There is also BeardedSpice which is a Mac native menubar app, works with lots of websites, YouTube included.

http://beardedspice.github.io/