If the Cheatriots franchise Cassel, and they almost surely will since Brady isn't even close to being ready to return, and teams need to designate a franchise player by the end of February, they won't be able to trade him. His agent will be forced by the NFLPA to look for a deal averaging close to the franchise tag ($14.5M a year), a huge signing bonus, and the Cheatriots will want a 1st rounder.
That being said, I doubt they even look to trade him because Brady's future is in serious doubt. The fact that the franchise tag keeps Cassel away from the other AFC East teams is a by-product.