18+ content is not allowed on workshop and we'll be moderating it.
Again, you can use your models privately, but only you can see it. You could also use friend-only viewing so people on your friends list see it.
I am very aware of Tabletop and how they do it.
Creating a system like that is beyond the scope of what we want to do right now. Our model importer system uses our own external tool that imports and converts them into our own proprietary model format that we can load in real time on Unreal 4.
Unreal 4 does not natively support loading models in real time (unlike Unity, which Tabletop uses), this is something that we had to code ourselves. So for us to add that feature, we'd have to code in real-time OBJ parsing and rendering support. It's definitely not plug-in-play, there's a lot of work to be done to make that feature reality.
Which is why we've planned to rely on making our own proprietary model format.