Most of the 640x480 color PowerBooks (500c-series, 190cs, color Duos, 5300c) could be set at 640x400 in 16-bit "Thousands" color mode, or 640x480 8-bit "256" color mode. I think only the 165c and 180c were incapable of this.

They did this solely by not displaying the top and bottom 40 pixels, no stretching was attempted. (Essentially, you got 'letterboxing' in high-color mode.)