Well, based on what little I know, you wouldn't want to leave a person standing with their arms stretched out to their side for an extended period of time, as that would mess with their blood flow. And you certainly don't want to leave a person gagged while unattended.

I think your best bet is having him stretched out on a bed and tied down to that somehow. I read a delightful story not long ago about a man who asks a female friend to show him what she's into and she has him lie down, doesn't tie him down, and then tells him to get comfortable. When she's sure he's comfortable, she tells him not to move. There's a fair amount of psychological stuff to the story, but the longer he lies on the bed, the more uncomfortable it becomes.

So yeah. I think even a sadistic person could inflict a fair amount of discomfort on someone who starts out in a relatively comfortable position and leaves them there for hours and hours. You could even make that part of that woman's sadistic streak -- he's comfortable at first, but not so much after several hours in one position.