I'm using sqlitebck which provides similar functionality but instead of using a file name to store the backup it uses connection instances.
I find it very useful. Here is my use case: to run tests of an application that requires a database filled, I do a 'copy' of an existing sqlite backup in a ':memory:' database.
But with the proposed API, it is not possible to use the resulted ':memory:' database because it will be delete when the connection is closed.
So I propose to add the option to pass a connection instead of a filename.