I've seen a number of sites/videos online that describe a method for measuring the speed of light, using a microwave oven and a chocolate bar. For example, this video on youtube. The basic idea is to microwave the chocolate for a few seconds, and measure the wavelength as the distance between the resulting melty regions. Then, reading the microwave frequency (listed on the door or back of the machine), you solve $c=\lambda f$.
It's a very nice demonstration, but the problem is: the only reason the frequency $f$ can be found listed on the machine is because it was designed to produce that frequency. The microwave source (magnetron) is a resonant cavity, and by adjusting the dimensions, one can figure out the resulting frequency if the speed of light is known.
My question is, could this experiment actually be done to "discover" the speed of light, i.e. if we lived in a world where the value of $c$ was unknown, but somehow we knew how to generate microwaves? Or in determining $f$, would we have already necessarily discovered the value of $c$ by some other means?