Apple's iMovie saves its Projects and Events files as folders named "/Documents/Movies/iMovie\ Events", "/Documents/Movies/iMovie\ Projects". You are not able to change this location using the app, however, you can use iMovie to move these folders to an external drive. On that drive, both of these folders MUST be at the root, i.e. "/Volumes/Footage/iMovie\ Events" and "/Volumes/Footage/iMovie\ Projects". If you open iMovie while an external drive has these folders at root, iMovie will automatically add them to its event and project library.
Unfortunately, you can't do this trick with network volumes by default. You can do some tricks with symlinks but when using them across volumes they are unreliable and iMovie will loose track of information.
With a shell command in Terminal, you can turn this capability on by changing a hidden setting.
defaults write com.apple.iMovieApp allowNV -bool TRUE
Quit iMovie and restart.
This still works in iMovie 10.0.4 as of 2014-07-12.
Oldest origin of this setting that I've found so far is: http://carryflag.blogspot.com/2010/06/imovie-event-library-on-network-drive.html
I first heard of it via: http://the.taoofmac.com/space/apps/iMovie
Not working on 10.0.9, unless I'm missing something about where the iMovie Library folder needs to be