If you don't own the VPN server how is routing the send through a 3rd party VPN server any more private than sending it through a 3rd party relay (eg. Sendgrid, Mailgun, Amazon...)?
I've actually been interested in similar features but for very different reasons. I have my Cloudron hosted on a VPS, but I also have some services hosted at home on a RPi and NAS. I don't expose my home to the public internet except via a VPN, so connecting a Cloudron service to my home network could be useful.
I don't have a strong usecase for it right now though and I'm making due with a set of Docker containers forwarding ports over SSH. https://git.iamthefij.com/iamthefij/dockamole. I'm currently working on a Dockamole server Cloudron app that will allow me to access the Cloudron LDAP server remotely for SSO on other hosts. I also hoped I'd be able to access graphite so I could aggregate Cloudron metrics into my Grafana instance, but it doesn't seem like Graphite is reachable from a Cloudron app.