I was thinking about how photons formed in the centre of the Sun take quite a long time to exit the surface. And it occurred to me that at the end of a star's life (during a supernova, given that it's big enough) all the photons that had built up in the radiation zone would be emitted at that point. Is the resulting emission of all those built-up photons what produces the great source of light seen during a supernova, or is it simply a contributor?
First, the Sun will not end up as a supernova - only a star $>8$ times the mass of the Sun will end its life in that way.
You also have the wrong idea about "trapped light" (photons bouncing around and gradually working their way to the surface). Photons are constantly emitted and absorbed again and don't travel very far (compared to the radius of the Sun), except at the photosphere, where by definition, they can escape.
Thus there is no storage of photons. What we see in a supernova is just a photosphere that is much hotter and much larger than the Sun. That is why it is many orders of magnitude more luminous than the Sun.
The heating (and explosive expansion) of the supernova envelope is ultimately derived from a small fraction of the gravitational potential energy released by the collapse of the stellar core.
Edit: The photosphere of a core collapse supernova is in fact about the same temperature as the Sun during most of the decay phase. Type Ia supernovae are a bit hotter. The huge luminosity is because the photosphere is huge.