What would you think if the hook is slightly before the center if weight?
A friend of mine showed me a such device years ago
A short stick with a short rope attached with a small loop at the end
This loop goes in a hook that is mounted direction towards the back end od the dart, but is mounted before the center of weight.
Thus the dart has already the right orientation while swinging, and releases itself from the loop. This could for sure be done with your sling design also... It could aid in precision compared to the loop.
I will look for the dart for making a pic to support my explanations

Good fly!