In typical UCD we begin requirements with methods that enable us to capture the wants and needs and get a fair idea about the user's mental model. Which helps us in developing the requirements for the product or service design. How do we do the same in absence of user involvement?
For various reasons I have to deal with this issue almost constantly. What we do here is:
This is by no means an ideal situation. To be honest, it's often like herding cats - you will spend a lot of time keeping things simple, relying on heuristics and best practice. But start early and challenge often.