Thanks again everyone for your help and suggestions.
I finally made progress on this: it does appear to be either a JeeLib or JeeLib compatibility issue. I hadn’t realised the OEM project has its own JeeLib fork, so I was using master from the upstream branch. Whilst I had already tried an older version, it wasn’t old enough: going back further seems to have resulted in a working firmware as does using the version on OEM’s github.
So something between then and the current JeeLib has broken things. I can use the older JeeLib for now, although I would suggest that the OEM project put a note about compiling this sketch on the hardware page (RFM69Pi V3 - OpenEnergyMonitor Wiki) indicating which version of JeeLib should be used/has been tested.
If it’s useful I could potentially figure out what revision causes it to break, which might yield a patch to either the RFM69Pi sketch or JeeLib.
Thanks,
Andy.