You can already kinda do that via multiselect and then deselecting the exhibit itself again by pressing ctrl+click. Then you just have the scenery stuff left in the selection and can move/delete that....
Also being able to select and delete objects I have placed inside of exhibits without having to delete the whole box would be very useful.
...
This does however remind me of the other thing this game severely lacks especially considering the building aspect of it: a proper demolish tool. Take the JWE2, that one's pretty much perfect with its option to select a whol lot of different types of things it deletes when moved over the park. Wanna get rid of all? Select everything. Just want to remove a few trees? Only select those and the brush will touch nothing else etc. It's so useful and yet PZ still doesn't have one at all and makes you delete bins and benches manually one by one...