Looks like ESPN has a chromecast app
http://espn.go.com/watchespn/apps
Also looks like I'll have to do some fancy footwork to get it to work for me, looks like there are some regional restrictions. Their Android app specifically lists the providers is works with and won't even let me download it to my phone, I assume because it's branded to a Canadian provider!
Don't see the NBC one, it must only be visible during the game. Would be great if you could go back and watch it VOD. The NBC page you pointed me to is pretty badly done, pc got laggy when I opened it, and it caused Firefox to ramp up to 25% of my CPU! A bit too much active content there, boyos.