Have you tried to get in there in the evening and roost him? My guess is if he's roosting right on the line, you might be SOL if he's headed for the private ground, but if you keep an eye on him, he may roost a bit further into the public land, giving you a chance to get between his roost and where he's headed.
If you can really pattern him, you might find that he roosts in different spots for different weather. Maybe he roosts over water when its raining, again, giving you a chance to get between his roost and his strut zone, which I'd guess is the fields where he's headed.
Just a few ideas.