Here in Papua New Guinea, TV broadcasts in SD PAL, and my company produces some magazine style items for local news.
We normally send one camera (Canon XL h1) which can shoot HD up to 1920 x 1080 or SD. I am trying to work out a post production path which will allow the camera to shoot in HD (1920 x 1080 pixels) and then in post crop to a selection of 720 x 576 pixels (our SD) and then use that 720 x 576 image as an SD output (ie without the program trying to rebuild it to 1920 x 1080).
The purpose is so that we can shoot (in HD) a wide shot of,say, two people on a podium, then cut it and broadcast as 3 shots, being the wide shot (simple down-res of the whole shot) and digitally zoomed medium shots of the two people - but still using 720x576 of the originally captured pixels without the computer guessing where to add more pixels as happens with a traditional digital zoom.
We use CS4 Production Premium including Premiere, and if it is better done in After Effects or another part of the suite we can do that.
The easiest way to achieve what you want is to use SD sequence settings and use the motion/scale and position effect to size/zoom the HDV shots to your liking.
If this is progressive footage, then chances are Harm's suggestion will work quite well. If it is interlaced, however, the end result may look less-than-desirable and/or take a long time to process in Premiere.
I also find the process of setting / copying / pasting many motion & scale keyframes rather annoying. You want to considering making 3 different SD sequences and then nesting these in a Multi-Camera sequence.