(Forgive me for this being OT, I couldn't find a map help section.) Okay, so right now, I'm trying my luck at mapmaking. I can't figure out how to make command blocks trigger without a direct redstone current being pumped into it. I am using MCEdit for this map, and am familiar with MCEdit filters (Referring directly to SethBling's filters.) I intend to use this to make certain messages go through at the bottom without a tripwire.
I think what you want is a player detector. You should already have everything you need to make one. They tend to take up quite a bit of space so plan accordingly. If you want the messages to be one time only you may want to think about including a latch of some kind like RS NOR or such.
Check this thread for some examples. Use your search engine of choice to find more. SethBling has also done a lot with player detection and custom spawners. It shouldn't take long perusing his youtube channel to find examples.