I am a side-sleeper, but find that on the trail, I can fall asleep rather easily on my back inside the hammock after a long day of hiking, and camp chores.
Then, as I get up in the middle of the night, I will roll onto my side, or onto my stomach, and it is much more comfortable as I am still half asleep. It is not easy for me to fall asleep on my stomach right away, however (in the hammock) if I try it when I first lay down to sleep, or am taking an afternoon nap.
Then again, I do not sleep very well on my stomach right away when I ground camp, either. :)
How about packing a flask of Drambuie so that you can sleep in any position, and under any conditions you might face?