There now appears to be a 64-bit driver for the MF-4100 series on Linux, which I downloaded from here.
After installing the two .deb files (cups-common and the cndrvcups-ufr2), the printer driver was available in the Add Printer dialog. It still didn't work though, despite appearing to install, accept and complete jobs. The trick was revealed by the answer in this askubuntu question, to search for missing dependencies for the .so libraries in the driver source tree. I installed lib32stdc++6, but as Dan Menes also discovered, it was the 32-bit version of libxml2 (sudo apt-get install libxml2:i386) that finally gets the thing to work.