I'm not sure this has been used to feed monarchs, but I tried it today. I found a pitiful female monarch in the rain today, she was holding on to a blade of grass, soaking wet. I took it home and put it in a warm, dry cage. Using the needle method, I got it to feed of a (very roughly 10%) real maple syrup solution. It fed for about a minute. Now it's hanging, trying to dry out.
I used the syrup because I didn't have anything else around. It seemed to work.