how to download registries from mirror site?

[gongfarmer]

I am using neovim on a corporate intranet without general internet access.
There is a mirror site which provides access to GitHub projects and releases.

My problem is that Mason cannot download it's registry.
I am configuring mason to download assets from my corporate mirror like this:

require('mason').setup({
  log_level = vim.log.levels.DEBUG,
  providers = {
    -- omitting registry-api provider, because it connects to https://api.mason-registry.dev/api/github/mason-org/mason-registry/releases/latest
    "mason.providers.client",
  },
  github = {
    download_url_template = 'https://proxy.corporate.com/generic-github-releases/%s/releases/download/%s/%s',
  }
})

When I run :Mason, I get this message:

Uninstalled registries
Packages from the following registries are unavailable. Press C to install.
 - github.com/mason-org/mason-registry [uninstalled]
 - github.com/mason-org/mason-registry [uninstalled]
 - github.com/mason-org/mason-registry [uninstalled]

Registry installation failed with the following error:
  GitHubRegistrySource(repo=mason-org/mason-registry) failed to install: Failed to fetch latest registry version from GitHub API.

The mason.log file shows this:

[DEBUG Thu 11 May 2023 12:02:41 PM EDT] ...al/share/nvim/lazy/mason.nvim/lua/mason-core/process.lua:116: Spawning cmd="wget", spawn_opts={
  args = { "--header=Accept: application/vnd.github.v3+json; q=1.0, application/json; q=0.8", "--header=User-Agent: mason.nvim (+https://github.com/williamboman/mason.nvim)", "-nv", "-o", "/dev/null", "-O", "-", "--method=GET", "https://api.github.com/repos/mason-org/mason-registry/releases/latest" }
}
[DEBUG Thu 11 May 2023 12:02:41 PM EDT] ...al/share/nvim/lazy/mason.nvim/lua/mason-core/process.lua:162: Spawned with pid 1280754
[DEBUG Thu 11 May 2023 12:02:41 PM EDT] ...al/share/nvim/lazy/mason.nvim/lua/mason-core/process.lua:148: Job pid=1280754 exited with exit_code=4, signal=0
[ERROR Thu 11 May 2023 12:02:41 PM EDT] ...e/nvim/lazy/mason.nvim/lua/mason-core/providers/init.lua:75: Provider "github" "get_latest_release" failed: spawn: wget failed with exit code 4 and signal 0.
[ERROR Thu 11 May 2023 12:02:41 PM EDT] ...e/nvim/lazy/mason.nvim/lua/mason-core/providers/init.lua:86: No provider implementation succeeded for github.get_latest_release
[ERROR Thu 11 May 2023 12:02:41 PM EDT] ...im/lazy/mason.nvim/lua/mason-registry/sources/github.lua:181: Failed to install registry GitHubRegistrySource(repo=mason-org/mason-registry). "Failed to fetch latest registry version from GitHub API."

The solution is probably that I need to configure the registries during setup(), something like this:

  registries =  {
      "https://proxy.corporate.com/generic-github-releases/mason-org/mason-registry/releases/",
  },

However I can't get Mason to accept a specific URL for the registry.
If I prepend 'github:' to the url above, then Mason adds a GitHub url onto the front before downloading.
Prepending 'lua:' doesn't help either.
These are the only prefixes allowed by lua/mason-registry/sources/init.lua, anything else causes a 'Malformed registry id' error.

How can I configure Mason to download registries from a mirror site?