I've tried a bunch of methods to get this to work, and nothing seems to stick or work to my needs (I don't want to turn off auto updates). So, I've solved the problem with Applescript.
Basically, I created three launchers and told Finder to use those instead of the new (or old) iWork apps. Since there's no version conflict, the default app change sticks in Finder... and you can have both old and new iWork installed and auto-updating.
1. Download the scripts from here (Dropbox)
2. Unzip, and place anywhere (e.g., Documents, etc)
3. Get info on an iWork document, and under "Open with:", select "Other..." and browse for the script you just installed ("Number defaulter" for Numbers file, and so on).
4. Click "Change All..." and accept.
Voila! You can now double click iWorks file and have them open in iWorks.
For those interested (or wary of downloading scripts), here's the text of each script bundle. Just modify "Keynote.app" to "Numbers.app" or "Pages.app" and resave.
on opentheFileList
set oldiWorkPath to "/Applications/iWork '09/Keynote.app"
repeat with theFile in theFileList
tell applicationoldiWorkPath to opentheFile
end repeat
tell applicationoldiWorkPath to activate
end open