Hmmm, a bit of a strange question. I'm not sure I understand you completely... I don't have any experience with drawing invisible walls..
It might not need to be completely invisible to get the feeling it's invisible, such as having dotted lines and partly transparent..
By the way, is this map top down or isometric? I'm not sure if it would work, but if you drew the wall then put the Fill down to 0% and did spotlight or drop shadow or something.. I'm not sure.