... but as I recall, in one of their tweets, it has been mentioned that LPM *is* possible on IPS/LCD ...
Yes, this is correct. At least in some levels...
Some people say it is not possible on any IPS or LCD screens. But it is. Then why these people say it is not? Because there are no actual use for it.
The whole idea for LPM is that it can display a clock, for example. AMOLED is (among the other options) the best solution, because you're able to activate only those specific pixels you use. So e.g. in N9 when you have LPM option on and you see the clock, the whole screen isn't actually activated. It's only those specific pixels. That's the reason why you see the clock a bit 'pixelated'.
LCD and IPC screens do not allow the phone to use only specific pixels. The screen is either on or not. So you could use LPM on any screens, but that would mean you have to force the screen to stay on all the time.
Try it sometimes that you select completely black wallpaper and disable the automatic screen lock. Test how many hours it lasts until it runs all the way down. Not much.
But as seen on one of the Dillon's presentation videos when the screen is waked by double click it's much darker at first. And when Dillon touches it again it gets the whole brightness. So I think it's their way (or at least was around when that video was made) that if you want to see the time in the middle of the night you won't be blinded.
Hope this helps.