Natural language query processing
I don't know the relative sizes and the nature of your problem, these details could completely change the view. But generally, if the only thing you have at your disposal is a small set of test documents, I would not recommend using that for term weighting at all. A small set of documents would give you an illusion of coverage, a skewed set of weights that would not accurately cover the domain.
If you do have general access to the IR system, though, you could try using the system itself for obtaining statistics by hitting it with random words and recording the number of search results, for example.