Personally I haven't got a clue what a 'handbrake cable compensator' is or were to find a 'rivnut' but I am able to report that the handbrake issue on my ARP4 was eventually resolved by the simple expedient of fitting softer brake pads. The handbrake now passes the MOT test. Nice. Thanks to Krazy Horse as ever.
Yes, I am sure that works fine, but the downside is that under heavy braking, the rears will be doing more work than they were, perhaps, designed to do, with the danger of locking up the rears first, which is obviously not a good idea, especially in the wet on a corner etc etc. In normal and sensible road use, I'm sure it feels fine however.
Will your handbrake, with the softer pads lock up the rears when on the move, or is it still and 'only just holding' solution?
I don't suppose you know the make and number of the pads do you Bob? It's certainly worth a try for very little outlay, it may even be claimable under warranty?