How do you use a power meter to pace yourself during steep hill time trial events? The events in questions are no longer than one kilometer and have a gradient that fluctuates between 7% and 13%.
First thing to do would be to get some data about your max power outputs over different gradients &amp; conditions.  So you'll want to know what you can sustain over a 10% 3 km climb, or a 10k climb at 5%.  You'll also need to know how this fits into different length rides - you'll be able to output a higher average over 15 km than you would over 30 km.  

The relationship between power, gradient, cadence, gear and speed is complex.  Increasing power will cause your cadence to rise, thus speeding you up.  The steeper the gradient, the more resistance there will be, so it'll take more power to remain at a steady cadence (in the same gear).

Dividing your power by your weight will give you a watts/kg rating which is useful.  

Once you know all this data, you'll be able to look at a hill and know that, all else being equal, you'll give your best performance if you maintain, say, 4 w/kg for the climb.  You can then use your powermeter output to adjust your effort accordingly.

Edit:
Mind you, if I understand the event description correctly, I suspect that over that sort of distance you are best off just pushing on the pedals as hard as you can.