Since nobody with better expertise has replied, here are my 2 cents:
The best guide for creating a registry that I have seen is here.
I have tried this path somewhat unsuccessfully. I ended with a registry and entries in it that looked valid, but led to TOML errors that I could not parse.
When I try to replicate your steps, I get stuck at create_registry which has no binding in my setup.
One way of figuring out what may have gone wrong is to simply look at the registry files (that should be located in .jula/registries.
Hope this is somewhat helpful and curious to see how this gets resolved.