Heroku can display custom maintenance page:
heroku config:set MAINTENANCE_PAGE_URL=http://some_server/my_page.html
This requires presence of some website where the page will be always online.
Could the maintenance page be loaded into Heroku once and forever?
No, since there will be no way for the Heroku router to show it if there is a maintenance problem on Heroku.
The canonical way is to put that page on Amazon S3, or any other provider that can serve static pages.