Currently it is hard to use addons that make use of Native Messaging. I suggest to add a readonly content plug so other snaps can provide the necessary files for Native Messaging. Something like
plugs: native_messaging_hosts: content: native_messaging_hosts target: $SNAP_USER_COMMON/.mozilla/native-messaging-hosts
This is not perfect. It means that there can only be one addon that uses Native Messaging at a time. Also according to https://developer.chrome.com/extensions/nativeMessaging
path does not support relative paths. That makes it hard to set the correct path for the executable. But I think having this plug is better than having no way to use another snap for Native Messaging.