For the benefit of anyone in the same situation who might stumble on this, I eventually managed to get my clunky old vu+ duo working as a remote client using OpenATV 7.3. The fallback tuner part worked great from the outset, but remotechannel streamconverter didn't work properly (for me) so I tried the Channels Importer plugin instead. That works fine and pulls the epg data from my main receiver with no problems.
It is a workable solution, but I would still prefer to use OpenVix as I don't really like the look and feel of OpenATV. Many thanks to Twol for helping me try and get a more recent openvix onto my crappy old box. You are a very patient man and incredibly generous with your time. I genuinely appreciate the support.