As seen in your example, export only exports the variable named, not the entire environment.

Packing it into a dmg means any OS X extended file attributes and/or resource forks are preserved and you can serve it from a non-OS X webserver. It is also why the package format was updated to include flat files in recent versions of OS X.

May I ask why you want to do this? It seems like a horrible idea - even a high performance SD card isn't going to be as fast as a hard drive, and having a non-standard configuration that wasn't tested by Apple is just begging for trouble.