Usually you would do that with a custom Prism plugin in this directory: C:\Prism\Plugins\Custom
There is a readme in this directory which briefly explains how to create a new plugin. In your plugin you can put your python command in the "onSaveFile" or in the "onPublish" function. Unfortunately there is no function in the plugin which gets called when you open a scene, but I will add that as soon as possible and let you know. I also plan to simplify the plugin creation in the next update.
Usually you would do that with a custom Prism plugin in this directory: C:\Prism\Plugins\Custom
There is a readme in this directory which briefly explains how to create a new plugin. In your plugin you can put your python command in the "onSaveFile" or in the "onPublish" function. Unfortunately there is no function in the plugin which gets called when you open a scene, but I will add that as soon as possible and let you know. I also plan to simplify the plugin creation in the next update.