Here's how this lockscreen actually works.

Replacing the stock lockscreen is near impossible, unless someone wants to rewrite the systemui plugin that handles tk (touchscreen/keyboard) lock, this involves using the same DBus signals and reading the configuration from mce.ini.
Why? Because someone felt it was a good idea to use the same plugin that handles locking and blanking to show the lockscreen UI.

This lockscreen pretty much works in the same way the stock lockscreen works, it displays over all windows (HILDON_STACKING_LAYER), but this one shows over the stock lockscreen.

Qt windows are slower than Gtk ones at redrawing, as a workaround, I show the lockscreen window right when you lock the screen, that way it has all the time it needs to draw the window while the screen's locked, if the device is unlocked, I simply hide the window.

The lockscreen actually listens to call status, if the device is not in call state "none", which is ringing, or call active or whatever, the lockscreen will never show.
I'm guessing the problem with calls is that the N900 kinda chokes when there's an incoming call. I'll have to look for workarounds.