If Cassel is franchised, he's done. There's nothing he can do other than refusing to sign the tender. If he does that, he doesn't get paid. Next year he can be tendered again. In theory, a team could franchise him yet again. Before Cassel can get his 'freedom', there will either be a new CBA in place or there will be no football with the NFLPA. The ridiculous jumps in the salary cap under the 'current' CBA have given teams the kind of money that will allow them to survive with 2 high priced quarterbacks if they feel a need to.
New England holds the cards. They didn't have the same kind of leverage over Samuel because he was a starter that they needed in the lineup. Cassel will be the backup, so every week he isn't there will be a week's worth of money saved by Mr. Kraft. If he's traded, the power to keep him for years will reside in his new team, Cassel will have lost the ability to have any say in his destination, and the new team will still control his rights for years.
As for the notion that the Patriots will be constrained in free agency, I can only suggest that you take a look at how little they'll be losing this offseason while still having about $8 million in cap space even if Cassel's taking up the $14 million and point out to you that there will be plenty of players still under contract who can always be approached about restructuring and/or extensions.