Since in the project this was for we didn't need a personal folder for every user in the system I created a user-folder page that offers a link to create a new folder when a user visits it that has no personal folder yet.
When there is already a personal folder for the current user then the user-folder page displays that user's folder using rp_internal_redirect. Thus that page can be referenced from e.g. /pvt/home as "Your Personal Folder".
Is this something that would be considered useful for general use? I could upload it as patch to file-storage (parametrized and off by default).
It looks like this feature simply was simply left out in the port from 3.x to 4.x - or is there some other part of openacs that's better suited to serve this functionality?