Hey Lawrence,
Great question! Not sure if you can do that specifically, but how about reversing the search?
For instance, start by finding all Random Containers and Blend Containers:
$ from type RandomSequenceContainer, blendContainer
... then copy the above list into a spreadsheet and search for all containers with reference to an Event Action:
$ from type RandomSequenceContainer, blendContainer select referencesto where type = "action"
... and then go back to the target containers of the actions:
$ from type RandomSequenceContainer, blendContainer select referencesto where type = "action" select target
From here, you could probably just filter the list of containers assigned to Event Actions, with the full list of random/blend containers in e.g. Google Sheet. Maybe something along the lines of...
=filter(A2:A, ISNA(MATCH($A2:A,$B$2:$B,0)))
... where A is the full list and B is the containers assigned to Event Actions.
If you're trying to clean out your Wwise project, here are a few other possibly useful WAQL sentences.
Get all empty Random or Sequence Containers: $ from type RandomSequenceContainer where childrenCount < 1
Get all Events with no Actions: $ from type Event where childrenCount < 1
Hope this works for you!