/echo is a command that you (or your script) issue to tell mIRC where to put a certainly line of data. What that means every /echo that occurs do so because you have told mIRC to do it explicitly. That being the case, all you need do is search through your script and change them all to /echo to a certain window (if that is your preference).
However, I believe you will find that you do not really want that, you simply want a certain type of message to go to a single custom window, such as perhaps CTCPs or notices sent directly to you (rather than a channel notice of some kind).
