Apologies. For the benefit of others, I should have updated this thread with my findings for anyone who should stumble across this via a search.
I did purchase an iPod Classic as a result of this thread. At the time, it didn't work with my existing version of MC (v14) but I downloaded a trial of MC17 and that sync'd perfectly once I had initialised the iPod within MC. I had no issues; it was so smooth you wouldn't think that J River no longer supported. Licence purchased, needless to say.
More than a year on, it's still working fine. My music collection is now 90% FLAC and sync'ing still works like a dream. No significant issues - the only major pain point has been when I've forgotten to prevent iTunes running which has then corrupted the iPod db which then requires a re-initialise and a re-sync.
I'm a happy chappy
Dreading the next the upgrade of my player but it's likely to be a hardware choice i.e. a player that works with J River rather than me trying to find software that works with an iPod.