Since nobody seems to have a solution handy I should share what I learned thus far:
This setup is essentially like the *arr suite and therefore I can use a similar configuration. The docker container of the object storage should be able to use the vpn container as a network device.
I‘m not totally sure how to do it on the other side since the n-containers have two network connections each, vpn and outside world, not to be mixed, for security reasons. Its probably not even a big deal but now its nearly done.
Its not implemented yet so its all theoretical. I might update once its done.