That No. 8 ring is probably going to be a pain in the ass to cut through, and expect you'll have to cut it in 2 places-- unless you plan on grinding an opening wide enough for multiple webbing layers to slide out of. And without damaging any of the webbing in the process. Even if you don't visibly nick the webbing, keep in mind the heat you'll generate in the process (if you want to do it with any kind of reasonable speed), which could cause damage to the nylon that isn't obvious to your eye.
Once (if) you get the D-ring off, the webbing loop that's left behind is sewn in such a way that a slink won't be able to tunnel all the way through and still be able to attach safely to a riser. The webbing loop that's left over was constructed in a shape to hold a long perpendicular bar of metal, not something that needs to assume the shape of a slink loop. You're constrained by stitching and the confluence wrap below the web loop, and you're not going to be able to re-shape the webbing to make that "tunnel" shorter. Even if you could thread some kind of material all the way through and connect if to the riser, worry about edge abrasion and adverse force angles deforming and stressing everything badly.
I imagine in the absolute best case fantasy scenario, you are going to be stuck with having to use L-bars and getting new or modified risers to fit them. But not being a rigger, I couldn't even say for sure if you could re-use the harness loop for that configuration.
What is more likely to happen is you will only get a partial cut through the steel ring before you give up, probably compromise the nylon itself, and now you are forced to send it out for a very involved repair, probably outweighing the cost of a new harness. (at least it was, last time I asked for a quote for harness work several years ago).
Better idea: get an old junk harness to practice your idea on, take it apart and study how it's built, prove me wrong, and then attempt it on your nice gear.
Best idea: get a quote from a master (not a senior) rigger before making any permanent changes to anything. It's a major alteration to the harness so a master should do it, in theory.
Colm, I've practically looked on my rig at all the points you've made, one by one and you are right especially that webbing, confluence wrap thing. It's just never going to be able to be shaped to accept the curvature of the slink. It can't work, dam it!