Selfhosted

Could you detail how you would do this?

I would re-read all docs about podman networking, different network modes, experiment with systemd PrivateNetwork option, re-read some basic about network namespaces, etc ;) I have no precise guide as I've never attempted it, so I would do some research, trial and error, take notes, etc, which is the stage you're at.

Edit: https://www.cloudnull.io/2019/04/running-services-in-network-name-spaces-with-systemd/,https://gist.github.com/rohan-molloy/35d5ccf03e4e6cbd03c3c45528775ab3, ...

Could you confirm if one can reach one’s containers on the loopback address in a separate network namespace on podman?

I think each pod uses its own network namespace [1]. You should check the docs and experiment (ip netns, ip addr, ip link, ip route...).

I think it's doable, but pretty much uncharted territory - at least the docs for basic building blocks exist, but I've never come across a real world example of how to do this. So if you go this way, you will be on your own debugging, documenting and maintaining the system and fixing it when it breaks. It will be an interesting learning experiment though, hope you can document and share the outcome. Good luck!