I strongly doubt that it's a one-play dynamic. I bet that when supplemental content is unlocked, it's unlocked and always accessible regardless of play-through.

This whole conversation makes me start to think about the proliferation of Wi-Fi on Nintendo and Sony devices. I wonder if publishers may even try to start limiting how many devices you can play a game on. I bet you the game can have a script in it to grab the serial number of the device and write it to the "save" file. Now that's just a matter of Nintendo or Sony allowing publishers to do that. Hopefully they won't allow that. Now, that approach can severely affect the selling of used video games.