Eliezer says on r/hpmor that it is a reference to the John Brown of Harpers Ferry. Responding to someone pointing out time-and-place difficulties, he says:

It's not to be taken that our John Brown was Dumbledore's student. Instead there was some other John Brown who, moved by the terrible suffering of a hellhole under the control of some Dark Wizard, went and attacked the Harp of Faery in an attempt to free them all at once - and died in the attempt.