They are intending to implement this in patch 1.19.
Read on:
http://forum.square-enix.com/ffxiv/threads/286

This is a bad idea imo.. Tethering causing you to loose clam is going to cause issues. Some NMs in specific require you to take them to places that they may very well tether from. Like Mosshorn, You have to take him somewhere with your back to a wall and sometimes he tethers.

If Some other jerkarse LS decides to be jerkarses "which they will" then they can steal that claim from your LS, even if the mob is at 5%. Tethering will cause you to loose claim.

I hope they rethink this approach or something. Looks like all kinds of fail waiting to happen.. Don't they test this stuff before they use it? I mean come on..