Presuming it's true that one can re-whiten ivories in sunlight, and
that placing the instrument in a sunny window will do that, consider
that the ultraviolet radiation in the sunlight will wreak havoc with
the finishes, tends to bleach the color out of wood, and the
temperature swings will be tough on the instrument.

I'd suggest pulling the keyboard and placing it out in strong sun,
or at least masking the instrument to keep the light off the varnishes,
and reduce heating the instrument.

Douglas S. Heckrotte
Philadelphia, Pennsylvania