Just a thought, but not having any knowledge of the location in question quite happy to have my theory adjusted. This is more of a warning system than a fail safe system
If working in an area where signalling is AB or Axle counter, could a track circuit block be installed around the crossing, linked to a local power supply, that whenever the train enters section with gates open sirens and beacons go off to warn the person that the train is near? I understand a lot of these locations are countryside, but for something that would get so little use, a solar/battery type set up would be possible, and being interlocked with the gates it would not go off unnecessarily? I guess the only real risk would be that it could make users of this type of crossing complacent and not get permission to cross.