It would need to be a virtual filesystem presented as a USB mass storage device, though. I've thought about this for years, but don't know whether anyone's act

It would need to be a virtual filesystem presented as a USB mass storage device, though. I've thought about this for years, but don't know whether anyone's actually done it yet. Early iPods, etc. were definitely not powerful enough to handle this, but this should be cake for even modest smartphone hardware. Basically the VFS would present an interface to the music library, protect the library against mid-sync disconnects, etc. Likewise, the "host" computer wouldn't be able to screw up the device filesystem (or even know what the read FS type is), access non-music user data, or have low-level access to device storage.