Probably a long way off, but I can't help but imagine the kind of amazing applications something like this might have. If we could reliably capture and store the resulting positrons, then recombine them with electrons later to release the energy and harness it somehow, I'd imagine this could make for an energy storage system with some really interesting properties.
It is an interesting idea, but there are other ways, probably easier, to get the positrons. For example, some radioisotopes emit positrons as part of their natural radioactive decay process. See:
Sorry, doesn't work. Light still exerts pressure, so even if you could losslessly trap light in a storage container indefinitely, the container would explode long before the energy density became 'infinite'. Furthermore, the energy and momentum in light contributes to the gravitational field, so even if you could make an infinitely strong mirror-box, pumping too much energy into it would eventually result in collapsing into a black hole.
