I decided to test DDW's theory that a greater number of ships can hold the land bridges. I used create_unit to spawn a large number of ships, which I then packed as densly as possible in and around the straits. They failed to stop the Seleucids. Looks like we'll have to hold them off the old-fashioned way.

I notice Braden asked above about Molon, and was given an incorrect answer. He is not in Maronia as of the latest save, summer 255. He's just across the strait in Abydos.