As time has gone on (and looking at the blog, it's been a lot of time) I decided that it would be great if plcd somehow knew that two devices were linked at the device level (ie directly).
As this is an Insteon concept, I decided it was the PLM protocol's responsibility to inform the engine that switching device A on also meant that device B turned on as well. So I added the parameters to the configuration file as "PLM.LinkedTo" followed by an incrementing number, so "PLM.LinkedTo0" is the first, followed by "PLM.LinkedTo1" etc.