Just some thoughts here, Player ......
Back when WGLI 1290 existed on Long Island, they ran 'merely' three towers. Their main lobe, the daytime one, incinerated downtown Babylon and was heard well in Bermuda. They were 5000 watts day. The water path did the trick, of course ; point is (reiterating) it took just 5000 watts and three towers ......
During Hugo in 1989, WOKV 690 Jacksonville left their 50,000 watt omni day pattern on overnight as the hurricane menaced the nearby coast. I was listening in Philadelphia to them, this station formerly known as 'The Big Ape' in their Top 40 days.
Now that was nighttime and skip, not daytime groundwave or water path. The thought here is that a WRVA waiver to go omni, or a WBT waiver to go omni at those times would do the job of guarding the coast very nicely. The best currently-licensed bet, though, for the northeast, would be to allow WTOP/WFED the juice for that 100,000 watts (or at go omni), I would think .....
For the daytime waterpath reception (even to 'relatively' inland places such as Philly and Richmond) I'd think an emergency two-tower setup at Wanchese NC would cover the entire east coast. The pattern would be loose, and would look like the reciprocal of WTIC 1080's nighttime signal.
Or if you will, consider it a two-tower pattern like WWL or WBZ. Send the bulk of the power inland and still leave plenty for the water path places. The engineering folks here would be able to balance what kind of wattage would be needed, or what the ideal frequency would be ......
All of this assumes that anyone other than DXers would be tuned to such a beacon on an AM radio for ANY kind of information nowadays, lol .....