yeaaaaaaaaaaaa set the loop end to somewhere between the midpoint and endpoint of the sample and make sure you're in osc mode. positive LPOF values (0000-7FFF) will slide the "window" forward, increasing the duty cycle. negative (FFFF-8000) values will shift it back decreasing it but it won't do this if you're already at the beginning of the sample.
alternatively, set the loop start point to somewhere between 00000000 and the midpoint of the sample and you can use negative LPOF values to increase duty cycle.
or you can get freaky and change the start and end point, but you need your zero crossing in the middle like bleo said, or you'll get silence.
http://hexawe.net/compos/c6420/
^ my track on here has an example.