There is no wired solution that I'm aware of.
Whatever they say, no one cares about WiFi above 10,000 feet. Almost no one bothers to turn it off and indeed many airlines now offer WiFi internet. If there's a wireless solution, you'll have no trouble using it.
On the other hand, you should have no trouble getting 12 hours of entertainment (music or video) on the phone itself, so a whole external drive might be overkill.
The idea behind having the hard drive would be I would have my entire library at my disposal, not just the media that would fit on what space I have on my iPhone.
I'll have to look into the wifi restrictions for airlines. I know alot of them offer wifi but they might have different rules for people who bring on devices that broadcast wifi.