The article repeats that they would like to set up a canary server but can't. But they also link to an askubuntu answer which mentions these two settings:

    Update-Manager::Always-Include-Phased-Updates;
    APT::Get::Always-Include-Phased-Updates: True;

    Update-Manager::Never-Include-Phased-Updates;
    APT::Get::Never-Include-Phased-Updates: True;

Surely the best setup would be to set some canaries to always include phased updates, and the rest of the fleet to never include them (so you get them once they are at 100% rollout and no longer phased)?