So I have been unable to quickly confirm this on the post office directly, but the commonly cited rule online is quoted as 917.243(b).

If we can believe the multiple sources and the rule hasn't been changed, I recalled it a little incorrectly. Using such a letter as a label is legal but if they deem it an improper usage, like directly mailing a brick with the letter taped to it, they reserve the right to just dispose of it. No trouble for the sender though, so it can't hurt.