With Smart Collection Sets, This is how I would expect to work:
Create a Smart Collection Set under Collections Panel,
Now if we create a Collection within the panel and add a bunch of photos to it, Any Smart Collections created within the Smart Collection Set should consider photos within the collection set to apply the rules.
The current Workaround is to create a Collection Set, Create an Album with a specific unique name or the same as the collection set. Create a Smart collection with one of the criteria to choose photos only from the above-created collection within the collection set.
This will make the workflow even smoother in terms of organizing and managing collections.
This will basically help manage a project being worked on as a smart collection set under collections.