A script to automatically split an Aperture library
It doesn't seem possible through Automator, but it could be through a little complex Applescript…

My personal approach was first gather various projects by context in folders (e.g. abroad holidays) and then - for each of them - export its content as an external library every 5 years. Hope this can apply to you!

O.T.: I'd like to post this as a comment, but it seems that I don't have enough reputation…