I have added tagging functionality to python3-aur's  AUR.Aurploader module. You can use it to write your own automatic tagging scripts. The functionality is also included in the command-line utility "aurtomatic", which is available in the same package.
For those of you interested in trying another AUR helper, I have also resurrected bauerbill.
I have made a lot of (mostly internal) changes to my pacman- and AUR-related modules these last few days. Let me know if you find any bugs. General feedback is welcome as always.
 https://aur.archlinux.org/packages/python3-aur/  https://aur.archlinux.org/packages/bauerbill/  http://xyne.archlinux.ca/projects/bauerbill/  https://bbs.archlinux.org/viewtopic.php?id=205834