I think if you can figure out a way to get 250kbps (25KB/s) across the link you would be pretty much set.

Drive up to the node, send a command to reboot it into a firmware update preset, send the file up to the node, and then have it reboot and then switch it back to the normal mesh.

The file would probably take somewhere between a minute and two minutes to get across between your node and the node that's receiving the update.