If you can create the exact same rough/uncut synthetic diamond, and therefore can deterministically reproduce the same cut on each diamond, then the decision of how to cut that diamond becomes intellectual property and is potentially copywritable.
If de Beers really can't hold back the synthetic market, they'll find a way to take it over, and legally-enforced monopolies sure isn't a bad strategy.
The most important commercial cut (the brilliant) was described in Marcel Tolowsky's 1919 book Diamond Design, so would not be copyrightable without some tremendous legal gymnastics. De Beers could dream up some "improvement" to the brilliant cut and trademark it, but there really isn't much room for improvement. The cut that Tolowsky described is very, very close to the mathematical ideal for brilliance and fire. Most of the later optimisations in diamond modelling are in the public domain.
I'm with you so far.
Admittedly their marketing scheme is under a little more pressure with the various articles about their "interesting" past and price fixing schemes, but that's a drop in the bucket compared to their decades of propaganda.
They will find some way, openly or through any back channel they can find or manufacture, to control as much as they can. Plus or minus further monitoring and exposes.
A more realistic option is use nanoscale etching to get unique colors. Much like those blue butterfly wings that don't use pigment.
If de Beers wants to patent and copyright some bizarre foamy cut of a diamond that does some exotic thing, that's fine, but it's not going to be "a diamond ring" as most people think of them.