@jamesh - these have been granted, but there are no corresponding review-tools updates at this time.
If this all works out to plan, I think a tractable easy path forward is to add processes in place for requests that come in when people ask to use greedy plugs with auto-connect for their theme snaps. In this manner, while publishers could upload them, they won’t auto-connect. These processes might include publisher vetting, agreeing to the provided content slots (that would be part of the snap decl of course), requesting that only css, images, etc are shipped, and not shipping any commands, hooks or daemons (ie, your list in Auto-connect custom GTK themes to gtk-3-themes, gtk-2-themes and icon-themes). You could then recommend anything that has a snap declaration that meets your criteria (ie, this request).
We could implement a quick check to prompt for manual review anything that slots any of the slots from gtk-common-themes, but I’m not sure how useful that would be considering the above.