There's https://gitlab.alpinelinux.org/Cogitri/apk-polkit. It currently works for upgrading/adding/installing (etc.) packages, I'll only have to add functionality to send the status of a current operation over DBus (right now the server binary just prints it to stdout) and add functionality to send the list of of installed/available/upgradable packages via DBus.
For that I'll have to make a MR to GNOME Software where it calls the right functions I provide. That shouldn't be much effort though since I designed the API with GNOME Software in mind (so it's mostly just linking the right lib and mapping the right functions to what GNOME Software expects).
Ah, forgot to to reply apparently. The privilege escalation part works now for all functionality I currently need. I only need to adapt the dbus client API a little now and then it should hopefully be good to go for a first version by the weekend :)