Clever idea. Yes, the data are written at the end of the session. However, I would embed your password in an instruction item rather than by using the global password option in the Preferences. There are some data writing activities that occur before the latter but not the former.
Also, be aware that some item types do write data during the session--e.g., essay item data, fill in the blank data when requested by use of the file name parameter. There are also some initial files that are modified when the session starts such as currentsubjectinfo.txt and autostart.txt (if applicable). If you start each session one after the other, that should not be an issue.
Again, I think this is a clever solution but I would only recommend trying it on an experimental basis to see if it works for you and to be mindful that the content of your experiment could have an impact on its success (as mentioned above).
If you try it I would be curious to hear how it goes.