As a little experiment, can you try changing your catalog using the following command in a Terminal, then closing and opening the App Store?
sudo softwareupdate --set-catalog "
http://swdist.furcode.co/index-10.1...n-lion-snowleopard-leopard.merged-1.sucatalog"
Edit: If the URL doesn't show up,
copy it from here.
I tested it a few moments ago in an attempt to find a patchless way to update, also without breaking permissions and messing with the App Store, and should work in theory elsewhere.
Please let me know if it works; anyone else who is on PB3 (or earlier) and wants to try it can also do so, it will help quite a bit to determine the viability.
Hint: To reset the catalog to Apple's official one after updating, just use sudo softwareupdate --clear-catalog