Yes, I think it is possible for sure. In this video, you can see someone unispinning with his feet only.
Also, at Ottawa Unicycle Invasion '08, Émile M.B., Julien Renaud and I were trying no handed unispins, started from seat in. Unfortunatly, we had to stop because all of us racked ourselves on our seat.
If you’re just begining practicing unispins, I would say you should try them with your hands first, much easier !
That was more like a 90 with a 90 body varial, and he had to put his foot on the tire I think. Never the less it was still cool and he probably has managed the 180, just not on video.