I started this project a few months back and dropped it out of lack of interest. I was recently reminded that it was pretty much done, and decided to just finish it.
This allows setting a different border for each dimension through the use of Magic Strings. See the defaults in the config for examples.
Here is a short Gfy for those of you who need pix or no clix.
Thanks to tterrag, if the mod is installed clientside, the user will see a red circle when near the border.