Too bad Toyota didn't do that,
I believe that they did exactly that. Look at cvtroger's photos in this blog entry. You can see the slim steel hoop around the outside, with a pattern of staking marks.

then we could just buy the 12 little rubber/bushing parts, press out the old, then in with the new.
It appears that you can. Febest's part number is TAB-LCB. Although it gripes me to buy anything from these guys I ordered up a few sets to see what I get. I assume they'll need to be machined. Sooner or later the couplings will be NLA from Toyota, and we Previsti will need new options. And I've no idea when Febest will quit making the inserts. Perhaps they already have, and just have some inventory left.

Seems like Febest had a good idea, but it was executed/assembled with complete incompetence. It's interesting how they are using bushings that are neither 12 mm or 14 .
Speculating wildly here, but they don't seem to be aware that the OD of the sleeve has to fit into anything. Since Toyota never made the rubber inserts a replacement part, and as far as I can tell never made the earlier couplings a replacement part, I suspect that they leaned towards forcing customers to buy an entire shaft to a) make more money, and b) control quality. There may have been minor mid-year changes as well. There's nothing in the FSM regarding the fit of the bolt sleeves into the shaft yokes. And they may have never made any detailed specifications available to the replacement part manufacturers, forcing them to do their own reverse-engineering, with varied results. As has been seen on this list, plenty can go wrong rebuilding one of these shafts. Same reason bicycle manufacturers got away from the loose-ball and cone bearings and went to a cartridge style. The performance is no longer dependent on the skill of some kid in a bike shop.

I'm impressed you were able to accurately reduce diameter. I have to admit I never checked hardness, but I assumed these bushings were hardened steel. Did you install this on a bolt & spin in a lathe?
I'm blessed to have an old Deckel FP1 with an index head. It's very versatile. Cutting the loose bits on a lathe will probably be easier, but I will need to make a bit of simple tooling. I'll give that a try when my batch of rubber bits arrive. The sleeves on the Febest parts machine easily - I assume they are just mild steel. The OEM sleeves are more precise, and may well be harder, but there's no need to machine them!