If we right-click on a slot and then the Choose Target pop up, a window displays: "Please choose" ... could we include an Add New... option when subsequently right-clicking one of the hierarchical object types, say a new Flow Fragment...
Then when later on we go back to Flow view or whatever, there'd be an interim palette of these new objects ready to place as desired.
Example UI image:
https://dl.dropboxusercontent.com/u/9344219/RMB1.png
Tom