On that, with bookworm (thanks @unman) I agree it seems that the better approach is to collaborate on extrepo and extrepo-data (landing in extrepo-data-offline package) to fill that goal. Out of the box, extrepo with extrepo-data-offline installed permits to install Signal repo data, but doesn’t contain element nor Session, while containing Syncthing (others I do not know)
The missing magic from dom0 or with cacher (accepting to modify the templates even more) would be to drop additional helper scripts in the templates it modifies so that a reapply-configure could modify the repositories definitions to be apt-cacher-ng compliant.
But yeah, extrepo seems to be the way to resolve https://forum.qubes-os.org/t/curl-proxy-wget-proxy-scripts-in-templates-so-users-can-add-gpg-distro-keys-linked-to-added-external-repositories if cacher could be helped a little bit into having those repositories and keys installed locally, and cacher_compliant_repo_changer applied, so users can just sudo apt install signal-desktop and others.