For Pete's sake. We're talking about K3s. 15-year-old devices. For the record, mine's been working "properly", unregistered -and- unstolen, for over a decade, leaving aside a while where I thought the battery had finally died.
Anyway, the reason I asked about it originally was that I thought it might be necessary because of the sudden
KUAL issue that people including me have been experiencing. I was trying various things to troubleshoot and thought registration might be part of the issue since my DXG has not had that issue and I'd had to fake register it - using instructions here on mobileread! - as part of the jailbreaking process, while I hadn't needed to do that for the K3. Seemed worth trying.
It looks like the KUAL/MKK problem is actually about certificates rather than registration so it's not an urgent question anymore.
But I still would be interested to know how to do it.