Part of the playtest agreement prohibits...wow, I actually just looked at the Playtest Agreement so I could quote the relevant sections and they've completely changed it. Apparently, Hasbro's lawyers got hold of it and turned it into legalese. According to the new wording, I actually can post my modified monsters, but they're instantly owned by Wizards of the Coast under work-for-hire provisions...which, as a writer currently working on a fantasy novel, actually scares me more than just "don't do it".