I doubt either of those could be made to work.
To get video over USB, you'd need a USB video capture device (quite cheap and readily available) and a USB host. MP3 players and phones are typically USB devices, not USB hosts.
A
quick google suggests that the G1 hardware may actually be host capable, but the software is not. Even if you managed to get around this, you'd need drivers to support your USB capture device, and enough CPU power (and bus bandwidth etc) to display it at a reasonable frame rate.
Assuming the line in on your MP3 player is audio (it appears to be), there is zero chance of that being of any use.
It would be far simpler to just buy a small NTSC/PAL capable LCD display. You can find some on sites dealextreme, ebay etc.