Yeah dlc is somewhat of an issue since you cant introduce new objects on the fly.
What you can do is a mix of things with what andreyin said, and heavy use of instances that you can change textures on via url, all though that method is pretty complicated.
Its doable, but will require quite a bit of forethought.
One thing that I find really interesting is that the problem of making objects on the fly is lessened if you use a server like nodejs that has no ties to graphical objects.