Odds are low but there are plenty of examples of solid QBs that have come from mid and later rounds.
Is Rattler that guy - doubt it. Though, a little spark of hope goes a long way around here. Heck, plenty on here were overly excited after our first two offensive showings of the year.
Some not to distant examples of reasonable talent...
Rich Gannon (4th)
Fitzmagick (7th)
Romo (UDFA)
Bulger (6th)
Hassleback (6th)
Brunell (5th)
Cousins (4th)
Dak (4th)
That being said I fully expect are QB for next year to be Carr (provided he is healthy).