I don't know how prevalent the practice was, but these are the first examples I've seen, at least from the Spanish American War era.
A pair of holographic telegrams datelined August 14, 1898, Madeira, Ohio, both regarding the death of "Harry", each with a 1-cent provisional overprint (Scott #R154) affixed. Nothing is written on the reverses.
Not the most aesthetically exciting documents in the world, but the first telegrams from that period I've seen.
