Organizational power comes in various forms. If an executive cares about the project and believes the junior PM is capable of running it, then that can be all the "power" that the PM needs to herd more senior engineers. If the engineers really have that much of a problem with it, they can go complain and be promptly told to stop complaining and get back to doing their job, i.e., contributing to the project.

As an aside, whether you're a PM or not, this is a good way to get promoted. On more than one occasion in my career, I've effectively led a project whose participants were on my boss's boss's staff. All I did was identify something that was strategic and important to the organization but that nobody at the next level currently had time to lead. I'd present the idea to my boss, then we'd present together to their boss, and I was in.