I don't think it's possible to replace the lock screen in a native way. All "custom lockscreens" in existence today are basically hacks -- you will notice flic

I don't think it's possible to replace the lock screen in a native way. All "custom lockscreens" in existence today are basically hacks -- you will notice flickers when the lock screen activity is initialized, for example. It's necessarily not customizable for some security reasons (unlock code, etc.)