If anyone would like to try this out early, here's some instructions for a build I just made.

Enable sideloading apps on your machine (see https://docs.microsoft.com/en-us/windows/uwp/get-started/enable-your-device-for-development - may already be done, sideload or developer mode should do)

In a regular (non-admin) PowerShell prompt in a temporary directory, run these commands:

iwr "https://dev.azure.com/Python/8e426817-76c0-4b99-ba9e-a48a1e4bd5db/_apis/build/builds/34540/artifacts?artifactName=appx&api-version=5.0-preview.5&%24format=zip" -outfile python37.zip
Expand-Archive .\python37.zip .
Add-AppxPackage appx\python.msix

(The first may take a while as it downloads approx. 80MB. I'm intending to leave the test suite out of the final package, which should bring it down closer to 40MB.)

Now play with it:
$> python.exe
$> python3.exe
$> python3.7.exe
$> pip.exe
$> pip3.exe
$> pip3.7.exe
$> idle.exe
$> idle3.exe
$> idle3.7.exe
(Or press Win+S and search for new entries)

To remove it:

Remove-AppxPackage (Get-AppxPackage *Python.3.7)
(delete the downloaded files)

Right now, the py.exe launcher can't find it, nor can any tools that rely on enumerating the registry - directly requesting the reg keys will return the right values though. VS Code seems to pick it up just fine though, I guess because it's on PATH.