assuming /cc 10, /cc 11 and /cc 12 work:
If you can not change the look of costumes after position 9, and it automatically defaults to the costume in the 0 position, then would it be possible to change position 0 costume to what you want for position 10, 11, and 12 right before you redeem/receive these costume slots? You still wouldn't be able to change them, but at least you could have 13 unique costumes.