I'm especially worried about UI thread getting blocked on file access to sleeping HDD or unreliable network drive (i.e. I don't want my applications to be as beachball-death-prone as Finder and iTunes)
Maybe use an actual network filesystem (NFS/Samba/whatever), and then throttle/shape the network traffic however you like (lots of suggestions upthread).
You could probably do it over a loopback device on the same machine if you don't have a network handy.
This has done wonders to show me just how buggy iTunes is, simply by trying to store all of my media on a remote machine.
Hm, I should add these mp3s to my library. Drag, drop, make coffee, hit the bathroom, chat with the QA guy, come back, read some HN comments, hey, it's done.