The average hour of television time includes about 15 minutes of commercial time. Providing an overlay, similar to that used for emergency or breaking news, at the foot of the commercials could provide the opportunity for people to stand up and jump around. The American Council on Exercise has several articles and PDF's that mention taking advantage of commercial breaks to engage in fitness activity.
The overlay could feature items like a countdown of time left for commercial, icons suggesting different activities (i.e. stretch, jog in place, leg lift etc).
Studies would have to be done to see how this would impact advertising but I believe that this will also insure that those participating remain in front of the TV, instead of wandering away for a break. Additional motivation for advertisers could be provided with a small text i.e. "This exercise break provided by: name-of-advertiser. We care about your health."
It seems to me that if you can't get people away from the TV, then leveraging their existing activity makes sense. You could also tie it into social media (Twitter etc) or other forum where people can share their success - and provide the possibility for viral media (i.e. 'our family lost 50 pounds working out during commercials!'), which would make this a wonderful tool.
Here are two small references from the American Council on Exercise, but the site is full of research and suggestions based on the problems caused by sedentary activity, with excessive television time being repeatedly cited as a major factor.
Couch Potatoes Arise! (pdf)
This study from the Bureau of Labor Statistics suggests that people view almost 3 hours of TV per day on average (the total may be up to 5 hours per day depending on age). 15 minutes of exercise, per one hour TV programming as suggested above, would result in 45 minutes per day, 16,400 minutes per year, or roughly 23 hours of extra exercise per month (or 38 hours per month for certain age groups).