I tended to use fixtures in order to save my data before propel-build-all commands. In a discussion on syncing development database structure with production, Mike mentioned he uses mysqldumps. This is actually not a bad idea, given that it is faster and less error prone (propel will never bug you). The downside is off course that it only works for mysql.
Here 3 Pake tasks to automate your mysql dumping:

  1. mysql-dump-data
  2. mysql-load-data
  3. propel-build-all-save-mysql

Download all three pake tasks. To install them simply copy the file to myproject/data/tasks/
You will have to edit the file to configure your database settings.
Note that this is only a temporary solution. I personally would be very happy to see Propel making database structure changes.
I didn’t see any manual on creating Pake tasks. Improvements and suggestions are more than welcome.