If interested in the UK, the Mr Clean brand Magic Eraser does not appear to be readily available but what I assume is the same product under a different brand is JML Magic Eraser which I obtained from a local Robert Dyas shop (apparently also available in Woollies and Wilkingsons shops).
Can't say I had as much success on my developer tray as the OP but it did take about 75% of the stubborn stains out (I always give a good rinse after use so any stains are well embedded in the plastic by now). Did about the same to a tray used for blue toning.
On the pack it says not to be used with bleach or other cleaners, or on black or high gloss surfaces; presumably the latter because of its abrasive nature.
Cheers, Bob.