OakLeaf-
From what I understand, you want to set pace alerts on something like a Forerunner, right? LIke have it beep at you if you go faster than a 9 min/mile? I would say that isn't a problem... in the half that I ran this morning I had pace alerts set on my forerunner for the exact same reason you say.... didn't want to go out to fast in the excitement and then not have enough left for the end. I set my max pace for a 9:30 mile, and when I would go faster than that it would beep at me once. It didn't beep constantly, just once when I started going faster than the set pace (this was good in the last 3 miles where I did start running faster since I felt good, and knew I would be able to hold it to the end).

I also heard lots of other people's GPSes set to beep at them, but it wasn't annoying or anything. There were also so many people and other noises around that you really only heard your own beeps.

Good luck on the 5 K! Have fun!