It might not be exactly what you are looking for but what I normally do prior to importing images:
Note: This requires Photoshop.
Hide flatten and hide layers until you have just the element \animation showing in Photoshop that you want to crop.
Navigate to Edit -> Trim
This will Trim just the truly transparent pixels as efficiently as possible
File -> Save For Web -> PNG -> Ensure transparency is checked
Then Undo in PS until you get back to desired state and repeat for the next animation/layer.
This works good from Spriter too when you are wiring up body parts.....