I assume you mean that you want to be able to add or remove music from an iPod/iPhone using just an external hard drive? If that's the case, it would more difficult than simply transferring the files to the iPod's hard disk. You'd also have to be able to add them to the iPod's music partition using the proper naming conventions and add entries to the iPod's library files (or whatever it is that it uses to keep track of things like play counts). You also have to keep in mind that it would likely screw up syncing with iTunes on a computer, since iTunes expects to be the only one adding songs to an iPod (at least until they start actually allowing you to download from the iTunes Wifi Music Store.
It sounds like it might be an interesting project, though.