I'm the one who did it. I used a program that simulated a Pro-Logoc II decoder, which extracts the center and surround information from a stereo source, much like a receiver would. It then saves each channel as a separate WAV file. These WAV files were fed into a Dolby Digital encoder, essentially storing the Pro-Logic information in AC3 form. It took months to research and perfect, but it sounds pretty good overall. Maybe I'll put up a short segment for people to listen to.

[EDIT] More specifically, the software simulated a Pro-Logic II reciever, which can give more of a "split surround" effect than regular Pro-Logic can. Technically, Pro-Logic II is specifically designed for stereo sources, but I figured what the hell.