I moved the code here if any interest.
https://gist.github.com/thierryvolpiatto/2219f99ac96ed1b468fac204bca23b4a

I added filters to register-preview (jump-to-register should show
only positions and insert-register should show only text).
Also added navigation (up and down) in preview buffer (see gif).

Still need to modify (or add) docstrings and also decide what to do with
`register-preview-delay` (just a flag in this patch).
AFAIU the only reason to delay preview is when executing kbd macro, in
this case we could use executing-kbd-macro, but maybe I miss something
and there is other reasons to delay this.

-- 
Thierry