A auto mod installer such as easy mod is in development for phpBB3 however I fail to see why you claim there is not automatic update procedure for phpBB3 when when of the update packages is called...and I quote
[ Automatic Update Packages (0/1-RC1 to 3.0.1) ]

This is a method of updating which will apply changes to the files automatically and only need human intervention if conflicts are found in the same way easymod would have when installing some mods