I'd found a lot of info on the net about using symlinks to allow iMovie to see network disks, but this always seemed like too much of an effort.
So this blog post was a real nice find.
Short form - you just need to set a default:
defaults write -app iMovie allowNV -bool true
And now - when iMovie is opened the network drives are just as available as any other - and you can simply drag to copy or move the events to your NAS from within iMovie - which keeps the links to projects working.
I'm moving off all except the last year - gaining me nearly a terabyte of space on my iMac back ;)