I agree a lot of what Sue has said and just want to add a few comments. I have had a few of my classes observed by student teachers or other teachers and they have all gave me some useful comments.
I think that after the demo that having a discussion is a great idea. If you don't know how the teachers think by this time then you will definitely will afterwards. Avoid lecturing them since you are younger and older Koreans hate to be lectured by younger people. Also try and get them to interact as much as possible and they could demo their own skills too and have others make comments on it. Since it's the end of the day, when you will have the training it's worth noting that many teachers will be tired so engaging their interest like this will encourage them a lot to pay attention!
Explain everything clearly and give them print outs of it. Expect to be critized (because you are younger and viewed as a newbie) so make sure you think of your answers.
About the amount of Korean they speak in class, demo a class where you speak no Korean and tell them that you don't need korean to speak. Sometimes I think teachers use this because they are lazy to "work the problem out." It's very easy for me to speak Korean when I want to explain something but it's not effective for the students. Also parents don't want their kids to be speaking Korean in an English class, they pay a lot of money and expect good results. It will be hard for the older teachers to change and in reality it's just a habit that they have to get out of.
I think showing different demos is a great idea. You can videotape your lesson and have them watch it or you can preform a "live" lesson. If there are any problems in the tape then don't stop it and just keep going with it - make it as real as possible.
Handing out questionnaires are a fast and realiable way of getting feedback. I think that leaving a comment box at the end of the questions will allow for the teachers to give you some extra information if they want to.
Hope that it goes well and let us know how you get on!
-Lik