People, locks and holds are easy to escape from, as long as you don’t let the person applying them put full pressure on!
Jerry,
If I read the original post correctly, the sleeper is not a simple hold. One arm is around your throat with the hand braced in the other elbow, which itself sits on top of your shoulder with its hand on the crown of your head. Not only is that hand there to stop you from head butting him (which doesn’t usually work anyway), it is there in case he wants to take you to the ground. All he has to do is drop down, and not only does he get a better lock, but you will pass out quicker. If he plummets to the ground, then your neck will break. If he so desires after taking you down, he can wrap his legs in a scissor lock around your waist. Now not only does he control your throat, he controls your diaphragm. You have no angle from which to reattack, and you have no power in your attacks, either. If he wants, he can kill you. Period.
In that case, if his plan is to take you to the ground and subdue you, then stomping his foot or knee will only make your situation that much worse, because he will want to sit down. His feet and knees have nothing to do with the strength behind the hold, so he doesn’t have to let go if he doesn’t want to. In fact, when training in this sort of stuff, people stomping my foot or knee makes me put the lock on tighter and lower my weight.
All he has to do is bury his face in your back and you can’t get to his eyes, which would not work anyway, even if you could get to them, which you can’t.
Striking or grabbing the groin is a good opener to get him to lessen the pressure, but does not always work. It works against me, but not against several people I know. So it may or may not work. Better to go with something you know will work, such as an elbow strike to or spear fingers to the ribs, which will definitely get his attention. If he has been holding you in this lock for a few seconds, then chances are he means to choke you standing, in which case, you have time to try one or two things. But best to use something that will get you out of that situation right away.
The elbow strike to the head is assuming that his head is at waist level. However, it will work just as well if his head is higher up, since it is going crossways and not straight down. In the second example, the alternative where you arms are pinned to your sides: the only way he can do this is to pin them above the elbows. Any lower down on the forearms, and he has a nothing hold, simply a hug around your waist. You can “strike” his abdomen with your tailbone easily from there. By the way, Wild Goose Leaves the Flock from Baguazhang works well for that one as well.
I can do these methods in a second or so. In fact, the quicker I do them, the more violent they become. If you can’t you need to work harder.
I always estimate the time it would take to put me unconcious at below 5 seconds. The reason is that you must react quickly, before the hold is on fully. Training to react with something that will at least lessen the pressure and distract him in that time limit will enable you to react effectively in a real fight.
“If all he is doing is holding, he can have my waist, I will be pummelling him.”
Well said, Jerry. 
“To enter is to be born, to retreat is to die.”
-An Old Taijiquan Saying