Thank you very much for the responses, guys! Quite right about the change in the production process for this top, I shouldn't have put it in the kiln with the ring on it - bad idea.
I have considered putting a flat ring into a rabbet to hide gapping and help support the edge ring, but rolling one accurately with my "hand mangle" would be very challenging indeed, and it would add cost to the customer. I have also considered making the ring in two or more parts, but because all boards are able to contract at the ends then isolated gaps are still possible IYSWIM.
I've looked at PEG but I don't have any of the required equipment (vats etc) and can't dispose of the waste easily/cheaply, and in any case it ought to be unnecessary given that I've had no other issues (YET!) apart from the very first table I built this way (which has just come back to me for the second time for a ring refit... I learned a lot from this one).
I've decided to halt production for a few weeks so that the timber that's in the kiln right now is dried to a point where it has little or no gradient, and I think this will produce the safest material to work with. Fingers crossed.
Does anyone have a suggestion for the least permeable finish available (in the UK if possible)? I'm thinking that if I can slow moisture exchange as much as it possible (by coating all surfaces including end grain with it) I can mitigate the risk of movement even further...
"After looking at the table top I read your name and smiled" - haha, well spotted ;-)