Just to clarify, you are looking for something that an on-field official can wear so they can communicate with someone in the stands for observation/training purposes.
What about a two way walkie talkie. Something like this:
http://www.bestbuy.com/site/olspage....=1099391907856
This may be a little bulky for what you are looking for. I've never used this product but it does say it has a hands free feature and comes with headsets. The wire for the headset would probably have to be fed down under the shirt. Plus you could purchase a couple pairs and communicate with every official on the field at the same time as long as they were on the same channel. Not a bad training idea.
Just a thought.