The problem with that is twofold. Putting 319's into the system in advance means shorting loads by approx 30 pax all winter long just to hedge against the few days a year you actually have to make fuel stops. The typical config for the 319 would be 134 pax compared to 156 for the 320. If you use that subfleet to cover for the rare days you have to make fuel stops, you'll have to bump up to 30 people. I think Jetblue is seeing significantly lower LFs since better yield mgt is sending the bottom feeders to other carriers. The 319 would give Jetblue choices as many of their routes will thin out due to the new yield mgt policies.
Besides how much longer range does the 319 have? The 319 has a range of 3700nm to 3000 for the 320. Do you have to get the ACT to make it longer range than the 320? There may be a few transcon markets that are thin enough year round to justify 319 service if it had significantly better range, but getting the 319 purely as headwind insurance doens't seem to add up. I think they limit pax loads in Burbank due to the short runway already. Especially if the acquisition and operating costs are almost identical, but the 319 having a higher CASM all the time.