So you're saying that if the source is 240p, but I'm trying to capture as 480i, there will never be any scanlines captured?That already is the original data straight from the TV.
In 240p, you're running approx. 60 frames per second interlaced with black lines to make up for the resolution, in 480i, you run the video slower, at approx. 30FPS, but with two "original frames" on-screen at the same time, they're interlaced with each other.
The amount of actual original frames is exactly the same - to get the 240p @ 60FPS video, you de-interlace the 480i video so that you isolate the two frames from one picture.
If so, I think that solves this.