Accept login policy banner without mouse
This appears to have been fixed in OS X 10.11 El Capitan, as the banner can be accepted using &#x21A9;.  Furthermore, the window appears fully on-screen in my scenario, rendering the need for a keyboard-only approach unnecessary for my use case.

So as far as I can tell, it is impossible to accept the policy banner without using the mouse in OS X 10.10 Yosemite, but they've fixed it in OS X 10.11 El Capitan.