There is no way to set the pixel aspect ratio that I can find.
However, set up a project to be DV anamorphic. Then click on a clip in your browser and then hit the info button on the far right of the screen. Click on the video tab at the top and then set the spatial conform to 'fill'. Every time you cut footage from this clip into the 16x9 project, it will be stretched to fill the frame. Needs to render though. Hope this helps. I don't know of a way to set all clips to 'fill' at once so you'd have to do each clip in turn.
As posted on another thread, I think I've found the solution to this.
You can purchase the application "QT Edit" which is part of the "Pro Media Tools" suite. There's a setting in there which enables you to chance the pixel aspect ratio without re-encoding. The latest (beta) version even has a batch processing feature so you can presumably change lots of files at once. Details here:http://www.digitalrebellion.com/
Alternatively, if you want to save $99 you can do the same thing manually using Apple's own "Atom Inspector" application which is free of charge if you sign up for a free Apple Developer Account. It's a bit fiddly, but not difficult to do. Full instructions are on this blog: http://alex4d.wordpress.com/2012/03/21/setting-_qt_aspect_ratio_flags/
However, while both solutions above fix the anamorphic flag in both QuickTime Player X and Final Cut Pro X, they fail to do so in Finder (and it's QuickLook feature - i.e. press the spacebar for a preview) or in QuickTIme Player 7 Pro. So there's one more thing you need to do, and that is go to the Movie Properties window, select the video track and change the Visual Settings so that the scaled size is 1024x576 (for PAL anyway). Note: you'll need to deselect 'Preserve Aspect Radio'. Then just save again.
Phew! Granted, a complete pain, but at least this fixes the problems we've been having.