Basically you'd need someway to advertise the storage device wirelessly. I think if you enable file sharing on the mini, so that other computers can wirelessly access it, you can choose the drive in the time machine selection pane, however if you have a wireless router you probably don't need to bother with this, an update over a year ago enabled support for NAS, so you can just plug it right into an airport (other routers should work too) and start backing up.