It would seem the answer to this question is...ask for permission.

It's been generally accepted by many in this community that works by others and modifications of works by others should not be distributed without permission of the creator/copyright holder. It's also been considered a courteous and polite thing to do whether there are legal issues involved or not.

I would be loath to do business with a merchant who overlooked the rights of a copyright holder in favor of adding value to his own products and gaining popularity, even if his intentions were altruistic.

While Joat Mon hasn't specifically forbidden modification, he did not expressly allow it either. I hope that Dodger will do the honorable thing and remove the corset from the file he's offering until he can establish that he has Joat Mon's permission.