As I understand it, stretched tuning in a piano is specifically intended to compensate for inharmonicity, not to compensate for the weaknesses of equal temperament. You can apply stretch to unevenly tempered pianos too, and it should also make those sound better. A formal description of stretched piano tuning is relatively recent (1938, with Railsback's publication of his eponymous curve), but some kind of stretched tuning likely goes back to the first piano, as it happens automatically if you tune the piano against itself by ear.