Coalesce all classes implementing a specified interface into a static list?

It depends on how you want to organize your code.

Probably the most straightforward way to do so would be to place a public method on your manager which would allow any object implementing the IHourlyUpdate interface to register itself when it becomes instantiated.

something like:

 public class HourlyUpdateManager
 {
     List<IHourlyUpdate> allUpdateObjects = new List<IHourlyUpdate>();
 
     public void RegisterUpdater(IHourlyUpdate obj)
     {
         if(allUpdateObjects.Contains(obj)) return;
          
         else allUpdateObjects.Add(obj);
     }
        
     void UpdateTick()
     {
         foreach(var obj in allUpdateObjects)
         {
             obj.HourUpdate();
         }
     }

each object implementing your interface would need to be responsible for finding the manager and registering. Or whatever object creates the interface would need to do so. That will depend entirely on how you're organizing things.