My first thought was a chain of custody with public/private key encryption for viewing the information. Perhaps file systems that record access?
Which is possible, but you have to lock out any viewers who can't demonstrate a secure chain from boot to viewing app (so you can trust the viewing app is "yours" rather than the device owners).
And there's always the analog hole. Unless you can rely on a 'Eurion' (http://en.wikipedia.org/wiki/EURion_constellation)-like mark to stop recording, people can always capture data at the point of display-to-retina.
The only other approach is the after-the-fact blamethrower, where you tag each copy steganographically so that you know who was the original source of any leaked or circulated copies. That wouldn't stop a Snowden though.
Who audits the software? etc.
You don't need any electronics for a film camera. Or any metal, for that matter. It's a risk to take such an item in, but maybe it's worth it for your use case.
And I guess if it's small amounts of text, there's always human memory (or writing on your skin?).
So you are (obviously I guess) trusting the people accessing the data to not abuse it, "helping" that trust with whatever technical countermeasures make it harder to move bulk data.
I pointed this out on the original HN discussion:
"When you cast a vote with Helios, you get a smart tracker to track your vote all the way to the tally. No one knows how you voted, but you can track your vote and everyone can check the tally. "