I've noticed them on some special JPs... They're a great way to definitely block a trem without relying on awkwardly-shaped woodblocks etc...
Are they public-available? If I ask nicely and say pretty please perhaps?
It's much more annoying to setup, and is not as efficient (e.g. there's still friction points, adjustments ot be made).
I'd get a tremol-no if I wanted to switch between hardtail/dive-only/floating, but I only ever need the hardtail mode.
(though I'm looking at it more closely now and it does look as if that plate requires drilling, vs. just resting against the cavity back edge, which negates some of my expected benefits. Maybe just the bog standard trem stop Tremolo Stoppers is the right way to do it for me then)