1967brutus

I would not use it that way. Break in is about temperature mainly, and keeping pressures and forces a bit limited. The rich setting does not serve to directly cool the engine or to lubricate it additionally, but to retard the ignition and keep forces, pressures and burn temperature down. So the rich setting indirectly results in cooler running. A hot plug advances ignition, and increases burn temps and pressures again so that more or less counteracts the richer setting, and in that case you're only wasting fuel.

Best to break the engine in on the same heatgrade and the same fuel that is recommended for normal use, and run it a touch richer according to the break in instructions, with a lighter prop to keep mechanical loads within limits.