The cable box s-video output shows whatever is on the cable box. Use the s-video input on the PVR-150 and there you go, record whatever the cable box is tuned to. Myth isn't going to "know" when to start/stop the recording, nor will it have the movie name/info, but once the video is captured you can edit, transcode, rename, etc. the video then use the web admin Amazon lookup to fill in the rest. I haven't recorded ppv stuff, but I use s-video to capture all the non-basic cable TV stuff I record.
Without starting a technical debate, in my experiences the PVR-150 s-video capture quality is roughly the same as watching TV via direct STB s-video to TV. In fact, I'd say it's a tad better than the PVR-150 TV-Tuner because the STB plays from a digital signal but the tuner is stuck with the analog cable source. It's still SD, so it sucks compared to a true HD picture.