Ed,
I suppose that there is a slight chance of that, however the fact that the engines have been seen at Croton-Harmon kind of defeats that theory. If they were sitting at Secaucus, that would make sense for a possible rescue role.
Croton-Harmon would not be a good rescue point however. First off, it's simply too far from Penn at a distance of 33 miles away. Even at track speed it takes close to 45 minutes to cover that ground. During a blackout, it would probably take at least 2 hours, maybe more depending on how many stalled Metro North trains were in the way.
Secondly, any Amtrak train running on that route would already be running behind a diesel engine, so it wouldn't need rescueing.
Finally, MN has its own diesel fleet and Croton Harmon is the main repair shop for diesels, so there is always extra power around.