Well, I never heard of anyone's doing Scarlatti molto rubato before. But I'm not a stickler for total tradition; if I were, I would insist that Scarlatti be played on a harpsichord. It does sound good on a harpsichord, but my aim would be to try to make the modern audience react the way the 18th century audience presumably reacted to Scarlatti's pyrotechnics, and for that, a piano is useful; a harpsichord, not indispensable. I don't think I would feel "authorized" to put so much rubato into baroque music, though.