> I am not sure what a 'post-installation script' means in practice.

IIRC, there's no way to prevent environment-variable expansion when shortcuts are created by an MSI, so the shortcuts need to be created or modified after installing IDLE. Perhaps this can be implemented in the MSI with a custom action. Or perhaps this can be implemented after installing the package via the OnExecutePackageComplete hook method in Tools/msi/bundle/bootstrap/PythonBootstrapperApplication.cpp.

> On the 'most recent' and alphabetical listings, one must right click,
> select 'more', and then 'open file location'.  One then gets a 
> directory of shortcuts, 

They're probably hiding the action to modify shortcuts since generally they're installed for all users in %ProgramData%, and modifying them requires administrator access.

> such as C:\Users\Terry\AppData\Roaming\Microsoft\Windows\Start 
> Menu\Programs\Python 3.7, which can be copied and edited.  

You shouldn't need to copy a shortcut in your own AppData. You can modify it directly.