I'd worry about gradual deterioration by using bleach on nice leather but I suppose if you diluted it enough, it wouldn't be a problem. I am thinking about getting a white saddle myself and wonder about keeping it nice. I would guess that precipitation (or more likely, perspiration) would be at the root of the problem but choosing a non-bleeding fabric should also be a priority.

Haven't tried the magic erasers but do notice that they easily take paint off walls, proof of their fairly aggressive abrasive qualities. I might have to try one on my tires though.