You can try creating a Junction to the "original" or "primary" Interfaces folder. A Junction is virtual directory that causes the filesystem in windows to present the folder as a normal folder, but read/write all content from a different location. So, you could delete the Interfaces folder on 3 of your 4 installs, and then create a junction to link back to the 1 remaining. This way, you only need to update one version of your AddOns to see the change in all your client installs.

There is a "Junction" tool distributed by sysinternals/winternals (now Microsoft) but I've never used it. I added symlinking support to one of my personal apps I use for 4-boxing my locks, so far I haven't had any problems with this approach (and load times improved significantly too.)

If anyone has the notion/time to properly junction/symlink/hardlink their wow installs, I highly suggest making the effort. It's SO worth it.