NOT SURE OF THIS BUT IF i can use wireless hard drive to play music mp3s and vids to my iphone now without hard ware update if spotify put software to let you save tracks lists or what ever to a dir on any net work on your iphone why would that not work.
i cant see it would take much to be able to save to a personal wireless hard drive.
I have all my music files mp3s saved on usb flash card and sd card pluged into my battey wireless drive, on my iphone it find the drive in wifi settings i connect to that, and run app that then conects to myhome wifi bridged, and with the app i can play my mp3s video, photos read txt docs all from my drive, if you can aford the memeoy you can have 2TB of storage, oh and from the usb you can plug in a usb hard drive.
it will do a lot more, but my point is could spotify add software to save to a wife address and save to a hd, sd card, or flash card.

Thanks for reading.

Leggoman