There wouldn't be a way to hand out access to your game early on. Mostly this is because development servers are not built to perform or handle the load that player access creates. But we are here to get people what they need to make money off of their products. So if you had a great looking game that had made real progress in art and engineering and level design, we can discuss providing an environment or an avenue by which the press could access your game. There are experienced teams, with games at a good stage of progress in our midst on the cloud who by special request have been given player client source code for their development world, for instance.
You would almost certainly need source code to build custom creation of assets into your game, but I could envision scaled down aspects that could be accomplished by some very knowledgable tech artists and engineers in the cloud using facegen and a number of different skeletons, meshes and textures, but there would never be a way for players to "upload" new content to the game space on the HeroCloud.