So you carry an external battery pack. The Magsafe ones for more recent iPhones are nice in that you don't even need to deal with a cable.
The laptop is an external battery pack…
In which case you connect it via USB, like you would with the dongle
Connecting your phone with via USB is exactly where the 'seamless' part falls apart. The comparison to dongle is also wrong, I don't use dongle for anything else, so I don't care if its not mobile