When I was young I was told that the dam at Keokuk, IA generated 25Hz power. Since construction was completed in 1913, this may well have been true. I was also told that the power was by then converted to60Hz and fed into the grid.
Whether I was told so, or figured out later, this must have been done with rotating machinery. I would guess, unless there is a cute hack I am unaware of, this would give a nasty waveform, but would probably be smoothed out by the rest of the grid.
The dam is still generating electricity, but the old generators and converters have probably long ago been replaced by native 60Hz generators.