@7675t so all is ready now to test. Thanks to @jdstrand pointing out my mistake in layout definition I was able to get it working on edge channel without any additional hack.
Ignore my previous post with bind mount, simply install my example avahi-client snap and you should be able to test ping on local domain
$ snap install avahi --edge $ snap install avahi-snap --edge $ snap set avahi host-name=core $ snap connect avahi-client:avahi-observe avahi; snap connect avahi-client:avahi-control avahi; snap connect avahi-client:network-observe core $ sudo avahi-client.ping core.local
Mind layout is fresh feature so snapcraft build does not support it yet and you will need to modify manually prime/meta/snap.yaml once you run build and add there:
layout: /etc/nsswitch.conf:` bind-file: $SNAP/nsswitch.conf`