Yes, it's possible.
What you call an "interface layer", is what we call a "repository layer". We always recommend our customers to hide the DevForce details behind the repository layers. Here's a write up on how to do that with DevForce-Caliburn Framework.
You can modify the entities access modifiers to internal in the EDMX designer. There's no need to make the EntityManager internal since as long as the entities are internal, they won't be accessible from the EntityManager outside the repository.
If you're interested, we have Professional Services team that can help you with guidance on best practices and architectural type questions.
Hope this helps.