I managed find a workaround, by downloading the files manually and then updating it through the App Store.
1. Start the update in App Store
2. Download the 4 following files:
http://swcdn.apple.com/content/downloads/57/32/031-68146/5t4lpv1g67z6nz8qih2qb97oqz46pccu9a/FirmwareUpdate.pkg
http://swcdn.apple.com/content/downloads/57/32/031-68146/5t4lpv1g67z6nz8qih2qb97oqz46pccu9a/FullBundleUpdate.pkg
http://swcdn.apple.com/content/downloads/57/32/031-68146/5t4lpv1g67z6nz8qih2qb97oqz46pccu9a/OSXUpd10.12PublicBeta.pkg
http://swcdn.apple.com/content/downloads/57/32/031-68146/5t4lpv1g67z6nz8qih2qb97oqz46pccu9a/OSXUpd10.12PublicBetaPatch.pkg
3. Move the 4 files into /Library/Updates/031-68146
4. Restart your Mac
5. Update through App Store
It should then install the Update like usual, this worked for me atleast.