Off hand, I remember the Patriots trading a 5th for Doug Gabriel sometime during 2006 season.
While that's not exactly at the same level of compensation as trading for Vincent Jackson, it still demonstrates that BB will make an in-season trade for a receiver if he feels that his current batch is lacking.
And anyway, as people have pointed out, precedent is kind of besides the point. Over the course of his coaching tenure in NE, BB's done just about every thing that the columnists and pundits previously swore he'd never do. The problem is that people are always trying to decipher general rules and principles from Belichick's various personnel decisions about which positions he values more than others, and where he likes to draft talent and where he likes to acquire veterans, etc. This ignores the one thing BB has said, over and over again -- every decision is about that specific set of circumstances, how that particular player can help that particular Patriots team.
That said, I think it's 10x more likely that the Patriots end up reaquiring Branch than trading for Jackson. Not only will he come cheaper in terms of both salary and draft picks, but he will also be able to contribute right away, given his familiarity with the Pats' offense and with Brady.