This works until new PLIST files (and the LOCK files) are created.

OS X automatically hides certain folders/files from the Finder, notably files that begin with a period/dot ("."). I thought /usr and some other areas might be treated the same way too, unless all of those folder have had "chflags" run on them.

Is there a way to add .PLIST.LOCK files to list of files that the Finder will always hide?

Lion makes a new .plist.lockfiles only when you install and run a new app which creates a new .plist preference file or if you run an existing app the first time in Lion. Once a .plist.lockfile is created by the OS, it isn't deleted and created again - it just sits there, visible or hidden. If you notice one or more new .plist.lockfile files in one of your Preferences folders, just run the script again ant it will hide the new files as well.