Unfortunately visibility is set at the Project-level (vs. Task-Level), so I don’t think it’s possible accomplish exactly what you’re looking for.
That said, could you create 2 separate projects (1 for internal work, 1 for client work) and multi-home Employee tasks from their Internal Project to the Client Project? That way, whatever is multi-homed to the Client Project is visible to the client, but you don’t need to give them access to the Employee Project with presumably a lot more tasks.