i don't think this is a good idea at all.
Sorry but having that application active will probably unlock your phone when it is in your pocket and you are moving about.

What is the point of having a locked screen when the phone is on it's back on the table anyway??
To be honest i don't care if it's locked or not in this position. The only thing i care about (like most of users i believe) is to have touchscreen and buttons locked when phone is in my pocket.
How can you guarantee that phone stays locked when you move about, get in and out of the car etc. At some point the phone will find itself in a position that is similar to holding phone in your hand and you cannot say how the proximity sensor will act while in the pocket so this won't help either. Then you will end up ringing random people or starting random apps.