Dockerized MongoDB on Heroku?
There is an official mongodb docker image which you can use. you just need to make sure you have docker installed on heroku.
If you are concerned about the data persistance you can easily mount host directories into your container so you will have physical access to your data. if you are worried about accebility you can easily expose ports inside your comtainer to your host so everything can connect to it.
Having your database in a container makes you able to be worried only on the db configuration and not the ehole stack . so when something goes down you always know where to look.