Bukkit

What is the enhancement in mind? How should it look and feel?
well when creating a border. it can create (using a switch) a block ring showing the border.
with a block you can set in the config
and when you change the border. the ring changes to. so that no old rings are stil there.

that would be nice to have
Please provide any additional information below.

I agree this would be a "nice to have".

Implementation wise, it should manifest as injecting the blocks into the stream sent to the player, not modifying the real world. ( Which will prevent the player actually placing anything on them )

And it would ideally have a customizable block type, such as for instance, the Barrier block , or simply something like red glass for visibility.

For visual reasons and reducing the number of blocks sent to the client, it may also be worthwhile to only send "border blocks" in the 1 block area normally filled with air/water above a solid block of any description, so that the border manifests as simply a perpetual fence, not a wall to the sky limit.

Of course, you will still be kicked back if you happen to fly or otherwise find a way to climb over that wall.