To answer your question, NO. A practice squad player-even an emergency QB, must be signed to the 53 man roster to be active for a game. I would imagine they would cut CJ Jones if they did that. Most here seem to think that if both Cassel and O'connell were to injured in the same game, K. Faulk would be taking the snaps.
Not that I would ever want to have that occur, but it would be interesting to see what would happen in that situation. A lot of QB draws, I would think.