Shingo Matsuura-san, a member of the Drone Japan School (which teaches ArduPilot software development here in Japan) developed this "Beacon Fence" feature on-top of our existing Pozyx beacon system support.
I gave it a try for the first time yesterday just before merging it to master and it works a-ok. You can see the vehicle stops about 1m before reaching the beacon.
During the test the vehicle was having some altitude hold issues that may be related to how the EKF is fusing the beacon data. We need to look into this further but it's unrelated to the beacon fence feature. No GPS was used during this test by the way.