It's also possible that adding a $200 retrofit heat purging economizer control on a high-mass boiler would cure the short-cycling on zone calls by making better use of the thermal mass of the boiler itself. With a heat purging controls a short call for heat to a small low thermal mass zone can be served by the residual heat resident in the boiler without the burners firing.
Adding a buffer tank to an existing system can be an expensive proposition, and isn't always economic or necessary.
A 15 minute minimum burn time is not necessary for efficiency. Even with a high mass boiler 10 minutes is more than enough. Burn times under 5 minutes with more than 5 burns and hour starts cutting into both efficiency and longevity. For low mass or modulating condensing boilers burn times as short as 3 minutes don't take much of a toll on efficiency, but you still need to keep the burns per hour bounded.
A TurboMax plumbed as the point of hydraulic separation can work, but has to be maintained at over 140F to guarantee that the water entering a cast iron boiler stays out of the condensing temperature range (as would any buffer tank plumbed as a hydraulic separator.)
^^That could be a TurboMax, or just a buffer tank with an aquastat control to call heat from the boiler. But either would be an expensive retrofit, so make sure you've ruled out the other options first.