I think the structure of their game, and Unity, might make things difficult. For one, you’ll probably need a Unity Pro license in order to export a .unity3d file which contains your level. For another, they would likely need to distribute some kind of project template which you can build off of in order to structure your level properly, because they had to do a ton of hacking in order to get Unity to even support the entire Rust island. Normally, you’re restricted to 10km maps in Unity due to floating point precision - beyond that you start to rapidly lose precision. However, I think they put a bunch of hacks in place which use double-precision floats, to increase the possible map size. This would probably make end-user mapmaking a lot harder.
Keep in mind Unity was really not designed to support user-created maps, or mods of any kind (one of the biggest shortcomings of the engine).