Well the simple answer is why not couple via an intermediate interface?
interface IMissionList
{
ObservableCollection<IMission> Missions{get;}
}
Then in your Dependency Injection bind that to something that can resolve to a instance, singleton or constant or etc...
then inject it into your Hud's constructor
Hud(IMissionList missionList){}
Now you are loosely coupled to a contract, the implementation of-which can be changed at any point, so long as the implementation adheres to the contract. Also, due to injection your Hud only has to concern it self with using IMissionList rather than also finding it.
[Edit] Sorry, this is C#, but hopefully the idea is of use. See for actionscript interfaces and Dependency Injection for Actionscript