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The problem with calling fileno() and fdopen() is that you bypass the buffering information held in BufferedIOReader. The first call works, but the FILE * pointer is now positioned at 4K, rather than just past the end of the object just read. The next call fails.
I verified that calling f.tell() after marshal.load(f) returns 4096, rather than just the size of the object read by the load().
Just to be clear, here's what I did in marshal_load:
 int is_file = 0;
 int fd;
 data = PyObject_CallMethod(f, "fileno", "");
 if (data == NULL)
 PyErr_Clear();
 else {
 fd = PyLong_AsLong(data);
 Py_DECREF(data);
 is_file = 1;
 }
 if (is_file) {
 rf.readable = NULL;
 rf.fp = fdopen(fd, "rb");
 }
 else {
 /* what I was doing before to set up rf */
 }
 /* and on to the read_object call */