I think this would be possible with Tasker, but I think the key to making it work would be a detectable holster. You can't turn the screen off with Tasker, but you can set the screen timeout to 7 seconds. So, when the phone is "docked" (in the holster) and the proximity sensor is activated, Tasker would set the timeout to 7 seconds. Another Tasker profile could then set the screen timeout back to normal once the screen turns off. This could work independent of the holster, but would then be using the proximity sensor whenever the screen is on.
Without a detectable holster, there would be no way of knowing when the device is removed from the holster, so it wouldn't turn on by itself.