When I say "progressive" (aka 576p) I mean all 576 lines that make up the full frame are together. Modern DVD players with component video connections can output 576p to a compatible display device.
A laserdisc player outputs 576i (interlaced), 2 fields with 288 lines each, and that's what my capture card samples. The video is stored that way on the disc, because it is an analogue video signal (although you could say that the original film print is progressive). When the capture is loaded in AVISynth, the 2 fields are automatically combined to retrieve the full frame - it doesn't throw away half the resolution! My mistake was thinking that I needed to do something else to join the two fields together before processing.
BTW there are some early (too dark) screenshots here:
http://www.originaltrilogy.com/forum/messageview.cfm?catid=9&threadid=1483