No this is a great question. The main step missing, is to give FilterWriter a sink filter to write to.
The following code creates a MemoryFilter as a sink, and then after it is filled with the TIFF data, we switch it to an source, and then for demonstration purposes we write to disk, but at this point you could give it a FilterReader and send the data to some other source.
pdftron.Filters.MemoryFilter mf = new pdftron.Filters.MemoryFilter(0, false); // zero, since we don't know how big the TIFF will be, and false = sink
FilterWriter fw = new FilterWriter(mf);
// now the MemoryFilter contains the TIFF data
mf.SetAsInputFilter(); // switch the MemoryFilter to a source
mf.WriteToFile(path_to_disk, false); // for demo purpose write to disk