It’s not possible, we have two FAQ entries related to this:
Can’t use CTB in production/staging: Troubleshooting - Strapi Developer Documentation
And can’t use CTB in PaaS style deployments: Troubleshooting - Strapi Developer Documentation
The same thing will happen in relation the database and file uploads because model schemas are stored on the file system, meaning making modifications within the deployed application on Heroku would write those model configs to the file system which will be reset when the dyno restarts.
Heroku’s File system is ephemeral and we have no plans to move the model configs to the database.