I wonder what the NFL's position on this is going to be. There's no way they can let Seymour dictate the trade terms. That would open a Pandora's Box of rebellion amongst the players. Don't like a trade? Just pout and don't go.
If the terms of the trade are valid, then the Pats will have just cause to say, as was pointed out, getting Seymour to Oakland is their problem. It's not the Pats' responsibility to hold his hand, put him on a plane, and then force him into a Raiders uni.
The key is the wording of NFL contracts, which stipulate that a player must pass a physical. Usually, the inference is that they pass it (and the trade is final) or they fail it (and the trade is nullified). As for a player REFUSING to take a physical? I don't know what they'd do.
If they allow the Pats to keep the pick, and Oakland gets screwed, then the NFL will have to deal with teams clamoring for new rules about trades which safeguard their side of the deal, which is almost as bad as having the players revolt and dictate trade terms.
Up until now, the NFL has been relying on the players to cool down, compose themselves, and ultimately go for the millions. If this drags out, it could get ugly.
What happens right now if, say, a trade is made but the player decides to retire? Does the team trading the player lose the pick they got? I suppose Seymour could screw the Pats by retiring, and then they're out both Seymour and the pick, but then Seymour's also out of a lucrative career.