Experiment with OpenNIC + Yggdrasil + YunoHost was successful in some sense. Managed to exchange emails between two YunoHost instances without any public IPv4 address, nor a traditional DNS domain.

However, the OpenNIC ACME server of course cannot contact a web server running on Yggdrasil, so no easy way of having TLS certificates.

Full writeup here: https://github.com/alexpdp7/alexpdp7/blob/master/linux/using_yggdrasil_opennic_yunohost.md