There wasn't (afaik) any special extension or drivers for the MacNexus and the FTDI drivers are up to date. Anyone ever tried this kind of thing with old serial gear?
Try find FTDI driver:
USBSerialConverter_040E2.sit.hqx
It is extension from 2004 and it should still work with MacOS8.6-9.2.2. And also might need Freemidi that it can check all available serial ports in machine (including USB-serial converter). I think you do not need special MacNexus driver, almost all mac serial MIDI gear support at least simple time piece protocol. But I don't have tested this kind of configuration, because I do not have those. Please test and come back how well it worked.