Chromecast: maximize tab on TV while multitasking on Chrome?
Have you tried creating a new space, opening a new Chrome window in that space, opening ESPN in that new window, and cast that single tab on that isolated Chrome window to your TV? Then you should be able to work in your other space(s) and leave that one alone/aside to stream to your TV.

I do something similar when I want to work in Chrome but stream Hulu/Netflix/whatever to my TV via AirPlay (I set the AirPlay display as a separate display and drag over a totally separate Chrome window to that AirPlay display).