Visually group custom and standard fields on screen
I found easier to move fields on the client side (using jQuery). you could write a simple extension that:


adds a custom javascript on the specific page(s) you want to modify
and that javascript would move the fields to their new location, and presumably hide the custom group "activity metadata"


It's mostly robust, but you would need to check everytime you do an upgrade that the selectors you used are still the same between different civicrm versions