Some Googling suggests that the answer is "no", but I might as well ask... Apparently the field is read-only to AppleScript, and if you hack the XML file directly, every "Date Added" will be set to the date at which the XML was re-parsed. (This means: "don't tell me what your guess is unless you've tried it and it works", ok?)
NOW, THEREFORE, I, GEORGE W. BUSH, President of the United States of America, do hereby proclaim July 30, 2006, as the 50th Anniversary of our National Motto, "In God We Trust." I call upon the people of the United States to observe this day with appropriate programs, ceremonies, and activities.
Apropos of nothing, it turns out that it's only illegal to write on or otherwise alter money if you do so in such a way and with the intent that it can't be recirculated.
How do I turn off the bit in a quicktime file that tells Quicktime Pro to gray out the "Save As" menu? (Yes, I already have a Quicktime Pro license; that's not what I'm asking.)
Now that I have dumped a bunch of downloaded videos into iTunes, I've discovered that I can't edit the names of the tracks on some files: things like 'Title' are stored in properties in the Quicktime file, and iTunes won't overwrite the file, so the title just reverts back.
I could just re-encode them with ffmpegX, but that's lossy, so I'd rather avoid it.