Hey Paul,
I'm sure you've long since finished or abandoned this project but I saw this post today and it seems to answer your original question:
http://forums.creativecow.net/thread/2/962945
I am script-o-phobic so I never used the DemoPalette.jsx panel before but there are some very useful utilities in there, which are very easy to use, including a script that scales an entire comp including cameras, layers, keyframes, etc. Just go to File > Scripts > DemoPalette.jsx. Click on the "?" button for a brief explanation of the scripts.
I tried running the "Scale Comp" script on a simple comp with a camera just to see if it worked and it seems to work pretty well. The only catch is that it will only scale in the original aspect ratio of the comp. So if you're scaling a standard 4:3 720x480 DV size comp up to 16:9 1920x1080 HD, then the script will only allow you to enter values that maintain the original aspect of 4:3. So if you enter 1920w and then enter 1080h it will automatically change the width back to 1620.
The way around this may be to just enter the width value and then after you've scaled the comp just go into your comp settings and change your comp height to 1080, which will crop the extra space on the top and bottom. Or you could let the script resize the comp to 1620x1080 and then expand the comp width to 1920 and fill in the extra area on the sides as needed. It just depends on your needs.
Also, don't forget to change the pixel aspect ratio to "square" in the comp settings and for all of your layers as well. I'm sure there's probably a script out there to automate that process as well.
-E
