I've come across a similar situation in a company where they work this way. Fortunately it was in a job interview and I refused to work there. But I was curious how it can actually works. For more than 10 years as market leader...
Anyway, I 've sticked with this for some time and studied different design approaches. And there is a solution for you! LeanUX is a way you should look for. It came from the Lean Startup movement and gets some traction right now. Its basically do some assumptions, concept it, prototype it and learn from users (aka do user research at the end). Due to the lack of resources of startups and a fast product launch, this approach has arrisen.
Smashing Magazine about LeanUX, and get a free chapter of Jeff Gothelfs coming book about LeanUX - its worth reading.
The same with personas, which can be quite time consuming to research and create. You can create them by good guesses of your stakeholders, which usally know their customers. Read this article about Proto-Personas.
Even if it's always better to go out and get a sense of end users by yourself - sometimes you cannot. But your assumptions don't have to be unprofessional.