Is it possible to get and set a Power Point document custom properties by manipulating it's OOXML?
I want to do so as Office.js does not have an API to edit PPT custom properties yet.
There's no API available to access the custom properties from within the add-in. Perhaps your scenario can be accomplished with settings or custom XML parts.
The only way to access the custom properties is to work with the OOXML using an app that is outside of the add-in. If the app is a web app, then it could be surfaced in an add-in; but it could not work on the very same document that the add-in is opened in.