How to move all iPhoto video files to an external disk or a NAS?
Try this (not automated, because i don't like automated delete for anything):


Make a "smart album" in the iPhoto with the condition "Photo is Movie". With this you get all movies into your new smart album.
press info (for check how much MB they're occupied)
Simply drag&amp;drop movies out of the iPhoto (long time)
ensure (double check) they are really OK
move the movies from the smart-album into the Trash
empty


(triple check before deleting - do not blame me, when your computer get destroyed or so)