Remote possibilities exist for placing a beacon but if weather was not VFR then such a backup to their plans is a valid possibility to consider.
Only a dumb@ss would trust a weather forecast that said clean and green for such a multiple target timed operation. And with their training I suspect IFR skills were lacking and such a signal or way point would'nt have done these pile-it's much good had the weather been overcast in NYC etc etc .....
Stay Safe
I wonder when Odtllah sold his gas station in Tennessee (hadn't the newsstand in the Newark train station where those two shaved guys worked been sold a few months before 9/11?) I wonder if Odttlah met Atta or the other ringleaders of the conspiracy in the weeks before 9/11.