David,
It has nothing to do with the size of the engine, but how the situation is handled.
By your logic then, across the board is it; OK to add water to a gas fired engine that is run dry? How about if the engine is coal fired?
Although we don't have the risk of critical crack length development in copper boilers (which makes things go boom), do you really want to instill the practice of getting away with it as being acceptable? I certainly do not. Complacency will develop and as a result it becomes standard practice to constantly stress items to failure, which is completely unnecessary and bad practice, period.
It is like running a automobile engine up past redline and saying it is OK to get away with it. Sure you can probably do it 100 times without issue, but what about 101 times? Every time the probability of a failure increases, even if you know the calculated risk and how to handle it, there is always a wild card factor that can make for an all around bad day.
I have seen smaller alcohol fired engines suffer similar problems under these circumstances. Yes, the larger burner areas can make more heat, but not without assistance. No matter how you cut it, running out of water is bad and correct practice dictates the heat source be extinguished to avoid damage to the boiler.
The line is so very thin between "getting away with it" and risking damage to paint, seals and/or boiler that it is not better better to err on the side of caution?
Of course the obvious thing to take out of this is don't run it out of water, period. It is not as if these boilers are so small that a safe water level cannot be maintained at all times of operation.