No further development - it works. It's one way only (ie the camera receives serial data) and the time window in which the camera is listening to the serial port is very short, but the throughput is good enough for sending simple commands to the camera. When I finally get the other components of my project done, I'll be sending aperture / shutter speed commands to two cameras to synchronize their settings and then be firing them in loose sync (Microfunguy is working on accurate [<10ms] sync of multiple cameras, I believe). That fairly simple brief is actually all I was hoping to do with this project.
I would be glad to help anyone else who wants to try and create anything that uses serial receiving. At some stage I'll do the civil thing and create a webpage with a sample schematic and code.