The reason the "package" hook for subiquity and ubiquity are included in apport is so that one can report a bug about the installation process after the installation has completed and subiquity and ubiquity are no longer installed. Testing the hook a bit I've discovered (remembered?) that the SourcePackage setting in the subiquity hook is there to workaround (or trick?) apport not starting the bug reporting process without that. If I just add "report['SourcePackage'] = ubuntu-desktop-installer" I'm able to report a bug about ubuntu-desktop-installer with or without the snap being installed. I'm going to assume that's the behavior you want and upload the hook with that change.
The reason the "package" hook for subiquity and ubiquity are included in apport is so that one can report a bug about the installation process after the installation has completed and subiquity and ubiquity are no longer installed. Testing the hook a bit I've discovered (remembered?) that the SourcePackage setting in the subiquity hook is there to workaround (or trick?) apport not starting the bug reporting process without that. If I just add "report[ 'SourcePackage' ] = ubuntu- desktop- installer" I'm able to report a bug about ubuntu- desktop- installer with or without the snap being installed. I'm going to assume that's the behavior you want and upload the hook with that change.