This is probably much easier than you think, assuming you two ended under amicable circumstances. If you two hate eachother, well, I'm guessing no amount of nudity is going to be worth having to spend time together.
Anyway, the best way to do this is to just ask, straight up. I'm assuming that you're a chick which is probably why you're asking instead of on the phone right now trying to convince your ex to just come over and hit you off one good time.
There's really no magic formula. Being as it is an ex, there's a comfort level and I'm sure you can call him up and talk to him about certain things you might be uncomfortable talking to others about. And this is definitely one of those things. I'm guessing he'll be more than okay with that as long as you all agree to not let it get funny (which it probably will - face it, movies get made about stuff like this for a reason).
So that's how you do it. Open up your mouth and ask if you can open up your mouth and hug.
It was written.