im not sure about this, but wouldnt the differences between PAL and NTSC give lightguns some trouble going back and forth between euro and us/jp as well as the deliberate incompatibilities built into HotD2? just a thought
If anything, it would be a software incompatibility, but I don't think that's likely to be a huge problem either. AFAIK, light guns/pens don't really have to care about the signal format; the gun/pen contains a photosensor that triggers an interrupt signal to the console, and the interrupt routine grabs the current beam position from the RAMDAC to see where the gun is pointed. In principle this should be mostly independent of refresh rate and certainly independent of color encoding. The compatibility issues with light guns come from displays that don't provide a detectable beam: projectors, projection screens, LCD, plasma, and in fact just about anything other than direct CRT.
He only mentioned the REFRESH RATE... which has little to do with pal/ntsc... if, say, the lightgun (or DC, or software, or whatever) works via a hardware timer that's independent of the refresh rate, and it times how soon after the last vertical blank interrupt the beam triggers the light gun, and the position is gathered that way, you have a problem if the timing is based on a 60Hz refresh rate but the beam actually takes longer to "get there" because the refresh rate is really 50Hz (slower drawing), then the DC/gun/game/whatever ends up thinking the beam position is much further ahead from where it really is. On one hand that means further down the screen, and on the other hand, an almost unpredictable left/right position...
EDIT: I dunno, I just have this feeling I just babbled some utter and complete nonsense up there. Don't bother correcting me, just ignore this..