That's why we don't let politicians design technical systems. They're bound to come up with solutions that sound good but that will not work in practice.
Some problems with your approach:
- partial loss of signal -> false positive -> power surge
- loss of signal -> false negative -> grid overload
- how do you signal the need for a little bit more power?
The British solution is typical: it's simple, and it 'just works', the speed with which hydro comes online it is just not enough to be reactive but even a little bit of forewarning and they'll be fine. Heck if the TV fails he can call home to mum, or some other place.
Solution of last resort: a slight brown out due to lack of information, but as long as this system has been in place you'd think if that were a statistically frequent thing that it would have happened by now.
Compare it with the auto-traffic-info system present in most car radios.
http://en.wikipedia.org/wiki/Radio_Data_System
The alternative frequencies system works along the lines you suggest (though technically it's not a 'frame' but an entire channel with its own carrier wave). This fails so frequently that I usually disable it. Either you get switched to some totally unrelated station (without traffic info to boot) and then never switched back or you're listening to a CD or so and then you'll suddenly be in the midst of some radio broadcast and again, you're never switched back.
Easy to propose, using an unreliable medium such as a TV or radio channel apparently very hard to get 100% right.
And if you're going to change major parameters in the electricity generation capacity of a nationwide grid I assume you'd put the bar for reliability a bit higher than I do for my car radio.