I haven't tested it for the moment, but isn't it possible to let C2 use dummy PNG files when you are working with it, and then, when exporting the final project, you switch the png file with your jpeg files. You can rename your png files and give them the name of the png files. For example, FF is intelligent enough to load and display an image with the wrong extension, because the image lib is reading the magic number at the beginning of the file.
It's quick and dirty, but it should work..