I've been searching Google trying to find a mod like this, and Google didn't get me anywhere useful!
I'm so glad I found this, and this needs to be added as part of the MIUI built-in code asap!
Darn. Thanks for the reply though cowsquad!
It looks like there isn't any way to disable MIUI's lockscreens either. So there rules out going back to WidgetLocker for now...
I've been trying to find this out as well.
R.E.L, did you ever find an answer for it? My search hasn't uncovered anything yet. I'm trying to do this with GoSMSPro popups.