The issue is that once you commit the transaction and it gets written to the WAL file, the ONLY way that I know of to 'undo' that transaction is to delete the WAL file, and the behavior of that is technically undefined. So yes, you can keep your main database from being updated, but every usage to read the data from it will see the data in the WAL file, so you haven't established a way to roll back to that last 'save' command. That would require doing something undocumented.