If it is really 30ft. away and there is no swingable option to reach the lead from a high TIP, then I would suggest setting an alternate TIP from the ground. Otherwise, you are going to have to use a throwline while in the tree or a pole to set the line while in the tree, both of which can be difficult but not impossible. Either option, to me, seems easier than aid climbing, which in effect is what you are talking about when using slings to climb a lead. It is easier to set a line (top rope) than to climb up a lead from the crotch with aid climbing techniques in my experience. However, I have been wrong before and don't mind being proved wrong again--it just means that there is an easier way than I am aware of, which is music to my ears!