I once saw and heard a tremolo attachment for an upright piano.
It consisted of a rounded box that fit on the back of the piano.

It contained a rotating fan similar to the ones that produce the
tremolo on an Austin pipe organ when their walk-in wind chests were
too large to tremolo the wind supply.  The rotating vane was run by
an electric motor and produced a very noticeable change in the tone.

The effect was, well, curious.  Probably wouldn't improve the
Waldstein sonata.

Bill Chapman