I've not come across it in French riding, but this is reminiscent of James Fillis, who was trained by a student of François Baucher. (For all his faults, Baucher's leg position is not as horrible.)

It's at best a brief phase, reminiscent of the old-style hunting seat (which one still occasionally spots in Horse and Hound), though there you'd expect shorter stirrups. Don't think I've ever seen a picture posed like this. The horse looks remarkably happy, given his seat.