Not truly possible.
the movement ini defines routes that exist for all time, and cannot be turned on and off like target areas.
the only way to do it, is how I've done it on the Cuba OTC+ , Okinawa and Philipines maps -- place the landing boats and offshore support craft either into a "city" target area (ie: Tacloban and Dulag) or a 'named beach head' target area that then can be tagged with "ActiveDate=" in the _targets ini (Beach Red, Green, Blue in Cuba)
But even doing so that way, fouls up any GroundAttack routes originating FROM the beach head, or defenders attaching said landings, when operating BEFORE their activation date
Be advised, as to naval gunfire support -- if you place the ships too close inland (less than 2 tiles offshore, approx. 5-10km), EVERY SINGLE GUN CAPABLE OF GROUND ENGAGEMENT WILL BE FIRING!!! with the attendant massive frame-rate loss. Went through this with both beach heads (Austin and Chrysler) on Kyushu.