Seeing things only from user side, isn't that what the proximity sensor is there to do in first place, to lock the screen when the phone is by your ear and you are talking to it :-). Can't code, so I cant comment to that part. I would use the reverse operation or at least test it, so I wouldn't have to lock the phone with slider when putting it back to my pocket in a hurry ...

Come to think of it, I think I wouldn't use the slider at all if the locking was reliable enough. Maybe it should lock the screen temporarily (if you uncover in this state, the screen opens back) almost instantly (say 1-2 sec) and make it permanent after 10 to 30 secs. So if you accidentally cover the sensor it wont affect the usage much, but it would lock the phone quick enough to avoid unintended operations. I Don't know if the system is responsive enough to accomplish this.