Sadly there isn't a way to change the Pivot Point of a object inside of the main screen.
Though, I do know of a workaround that may help you out...
1. Create a dummy object that will be grouped together with the object you want to turn.
2. With this grouping you can move the dummy object away from the original in order to "move" the central pivot point to the desired location.
I hope this helps, its a bit of a workaround but should get you a custom rotate pivot point.