There might be a workaround for this. I've been using "Hot Keyboard" macro for the past couple of years to deal with such problems.
It has a very neat keyboard recorder/editor and you can also set several conditions for running a macro with a keyboard shortcut. In this case you can set a keyboard sequence like this to get it done:
f(full screen),7tabs(get to setting),space(open setting menue)2down(get to quality),xdown(get to the quality you want),enter/space
Then you can assign a keyboard shortcut in 'Hot keyboard' to this sequence. Such solution works seamlessly for me most of the time as if I've added a costume keyboard shortcut to YouTube for myself.
BTW: The developer of 'Hot Keyboard' seems like a very nice guy. Don't forget to support him with a purchase.