I've heard stories, Some place in the midwest... some place in Quebec...
There was a time that I called every gear vendor I could find in North America, including Getrag NA they basically told me to get lost, they sold all that crap to someone else, I tried following up with that someone else... that was years ago now I forget who it was.
I've pulled all the diagrams of 284s. I have a broken one to experiment on when I find the time. But fundamentally, without a source for new blocking rings/synchronizers we're stuffed. Brearings, seals are relatively standard and can be obtained through one way or another.... unless we're talking input shaft seals. 284 ones are impossible to obtain, i'm sure that could be worked out though.
So basically no one is making the essential blocking rings or input shaft seals... so what can be done?
As I see it without original technical drawings we have no way of knowing exactly to what specification the blocking rings were made to. This could be somewhat overcome with disassembly of a known working 284 and you could get an idea of how worn the components are.
Now taking all that knowledge you could make cad files of the parts and find machine shops willing to do runs of the components, this is probably quite expensive and would require buy in from a bunch of people to make it worth the while. The upshot of that is most people only have issues with the second and third gear blockers going bad so that in theory 2 parts.
This is one of my many backburner projects, I'll get around to it one of these years.