I was brought here by a news article and like the idea. But your scepticism towards open sourcing the project makes me skeptical about your project :D
If sandboxing is hard, why not use the experience and intelligence of hundreds of developers out there? What needs to be done by you that cannot be done a lot faster by a crowd of enthusiasts?
Apart from that, you should consider dealing with adult content from the beginning. It should really be done during the design phase if it's not already too late. There's a huge debate about such topics for normal websites already on a government level and it could be a death penalty for the project if it cannot handle requirements properly.
Still, keep it up and I'm excited to see where this project is going!