Well that was unimaginably easy. It didn't completely seal the ball but this was much more straightforward than my attempt.

If you couldn't tell by the title of this project log, that £75 (used) ball chain splicer arrived. The scissor movement seems to greatly reduce the amount of effort required to pin-crush the ball off the chain. I had a feeling that I'd have to create a fully 3D printed tool, since the ratcheting feature of the SN-28B was going to be an issue.

I feel like an improvement of the design would be a retractable stopper, preventing the spike from fully crushing the ball and just opening the insert hole enough to feed the linkage into it.