i have an iPhone not an ipod touch, but if you go into artists and select a song it will play that song. i would suggest making the songs you want to do this to a special artist name that no other songs associate to. if you do that it will play the single track and stop.
I know this is an old post, but I had the exact same problem and wasn't able to find a solution either. I solved it myself by writing an app (for iPod/iPhone/iPad) called One Track Mind that plays one song at a time from a playlist. It also allows you to loop a song or start it with a delay. Check it out here: http://www.houseofpung.net/otm/