What you are requesting would require creating a separate app that contains the bundled map to be uploaded to Google Play Store. So every time someone would want to distribute a different map with Custom Maps, a new separate app (with a separate name and separate updates) would need to be added to the Play Store. That is not feasible.
The best you could do is give them one QR code pointing to the Custom Maps app in the Play Store, and another one pointing at the map file on a web server where it gets downloaded.
Marko Teittinen
Creator of Custom Maps