It's not so much legal agreements as a limit on what's said publicly, and of profitability. I can say with a good deal of certainty that IWM won't be interested in re-doing a mini that just came out, and that the Wasp is the size it is because TPTB said so. As Stinger has said, aim to do something that has not been done - there are a number of designs in TRO:Prototypes for example - to work on your skills. Put it this way: if you wanted a job with Company X, would you apply and demonstrate your capability, or would you barge into their office, tell them their product is crap and that you can do it better? Do you think the latter method will allow you to integrate with the current workforce if you were given a job with Company X?
In my experience with CSO, it's more often not a lack of skill or potential that holds people back. It's ego. What happens to your grand designs on revolutionizing IWM's product line when your first (and second, and third....) effort is rejected? What about when you're asked to re-work a piece you've done that's perfect to you to change proportions, details, and even scale? How about when your print is crap and you've got to back-track for hours to find the source of the problem, or re-do it completely? A beautiful 1st Ghost Regiment mini doesn't happen because you sit down with a picture of a tattoo you like and a paint brush, it happens because of the thousands of hours of painting you've done before then. Show that you're willing to work and take direction. And to be let down. Even as a sculptor for IWM, you don't get to just pick and choose which pieces you get to do and apply your own vision to them.