As far as I can glance from the source code, you can indeed only pass a String in. So your options are basically to either follow the idea of Josh Voigts in his comment or to implement something of a reverse IOString: something that has a String interface, but 'lazily' fetches the requested contents from a File.
I'm not entirely sure whether that is even possible without resorting to C and even then there may be methods whose semantics are such that they simply cannot be implemented consistently, but perhaps the subset of String methods used by Treetop is such that it is manageable. However, I'd say Josh Voigts answer is most pragmatic.