“But wrt to Robb, there is legal precedent for his will. The Night’s Watch has released men from their vows in the past.” When was this?

Ok already! Given the flood of anon asks, I’ll answer. So the evidence for this is as follows:

“Jon is a brother of the Night’s Watch, sworn to take no wife and hold no lands. Those who take the black serve for life.”
“So do the knights of the Kingsguard. That did not stop the Lannisters from stripping the white cloaks from Ser Barristan Selmy and Ser Boros Blount when they had no more use for them. If I send the Watch a hundred men in Jon’s place, I’ll wager they find some way to release him from his vows…”

“Not unless he’s legitimized by a royal decree,” said Robb. “There is more precedent for that than for releasing a Sworn Brother from his oath.” (Cat V, ASOS)

Granny: Is there any chance that Jon could be released from his oaths of the nightwatch?

George_RR_Martin: The great council would have released Aemon from his maester’s oath, so I suppose it would be possible. With an appropriate authority. (Source)

Let’s break this down:

•  Robb’s comment that “If I send the Watch a hundred men in Jon’s place, I’ll wager they find some way to release him from his vows” suggests to me that there is some process for the Watch to release someone from their vows in extraordinary circumstances. 
• Likewise, Robb’s comment that there is more precedent for legitimizing bastards than there is from releasing someone from their oath suggests to me that there is some, but very little, precedent for releasing someone from their vows.
• GRRM’s response to the question about Jon, and keep in mind this question is being asked after ACOK came out but before ASOS came out, suggests to me that the “appropriate authority” can release Jon from his vows. Now, the authority in question could be many things: the Lord Commander, the Brothers voting as a whole, a king’s decree, etc. 

Also, I remember GRRM talking about it on some HBO video, an Inside the Episode or History and Lore, where he said that it had happened but they don’t like doing it, but I couldn’t find what I was looking for.