Unfortunately editing the floor loops doesn’t allow you to “notch” the geometry.
However, using @jacob.small’s logic you can do this with spring nodes and clockwork. (I did have a little fun with the “notching” that I used.)
The workflow in Dynamo:
The Result in Revit:
And some sample files if anyone else wanted to have a go with it.
GenerateVoidFamily.dyn (24.7 KB) VoidFloorWithFamily.rvt (532 KB)