Original place is a directory. It must exists to be mounted. I’m not sure if we are talking past each other without understanding the point each person is trying to make.
As an example. Let’s say my system has an user with HOME set to /nas/user
As that user I wish to continue using that location. Moreover I wish to to use pathnames referring to files in my home directory when interacting with snap apps.
The problem is that from an app point of view /nas does not exist. The root filesystem is an immutable, read only core snap. To mount /nas/user as /nas/user inside the “container” I need to create /nas and then /nas/user but I cannot since EROFS.