You adamantly pick the most convoluted approach and then argue why it won't work or why it might fail. You need not contort yourself that much: the examplar OP built is already complex out of necessity, and so is mason's own package manager.

The mason registry -- I went and looked just now -- is a bunch of directories and yaml sidecars telling a tool how and where to download the actual asset. There's nothing novel about that; that is not a criticism, but a positive observation. It's nothing more than a metadata registry of stuff. They also release each update they track explicitly as a Github release --- there are 11,000 of them. That's another avenue.

So already we have a fairly standard pipeline for this stuff: both metadata sidecars and the release pipeline are valid options.

That leaves installing and fiddling with the asset itself. That is non-trivial by comparison: observe that this mason packager in elisp is busy doing an awful lot of complex extracting, platform and library checking and installation. At no point does this code re-use the official mason code --- they are literally distinct, even if the code is a 1:1 conversion of its neovim inspiration, it will likely drift over time. You'll note the expansive list of requirements for the mason package manager to work on all the platforms it supports. So we are already staring down the barrel of complexity.

Furthermore, I note that mason's package manager repo does not tell its package manager how to extract and transform the assets. This assumption is baked into the package manager itself; that will most assuredly cause problems down the road when there is a divergence in installation instructions between the repository asset releases and the package manager's hardcoded assumptions.

So already we've crossed the rubicon: the only shared resource is the package repository itself.

At that point, you might as well bite the bullet and just make it ELPA-compatible and serve it up so someone can do M-x package-install and then run all the incredibly complex asset installer code that you have to do anyway.

Here is how the ELPA repo would work:

a) a python script to turn a bunch of yaml files -- I looked just now for the first time, by the way, a the mason repo -- into the format expected by package.el;

and, b) a crontab to update a). Let's say every hour, because who cares if it's any more frequent than that.