@ijohnson, I am currently working on a cups-proxyd which mirrors the queues of the system’s cupsd to the Snap’s cupsd, so that the Snap’s cupsd has the same queues but passing the jobs unfiltered to the system’s cupsd (drivers are on the system’s cupsd).
I do not expand cups-browsed to use that, because operations here are well different, and complications as taking note of network interfaces or the implicitclass backend should be avoided. Also cups-browsed is supposed to get spun out of the CUPS Snap. cups-proxyd will then be run by the startup script for cupsd, no new “app” created in snapcraft.yaml.
There is only one problem which I discovered now with @alexmurray here, the “cups” plug for client Snaps only allows access to /run/cups/cups.sock not to the alternative socket in $SNAP_DATA, /var/snap/cups/common/run/cups.sock. This needs to be added, I am talking with @jamesh about this currently.