Programatically update serialized script variable

I need to get my application's version during runtime (Though this question would be useful for many other things as well).

This is possible in the Unity editor using PlayerSettings.bundleVersion, but the PlayerSettings library isn't available outside of the editor.

How do I update the default value of a variable serialized to a script while that script is running in the editor? (I can of course update that variable by hand in the inspector while the game isn't playing, but I love automating things to reduce the possibility of human error).