Map-making question

I'm making a new mod for Medieval featuring a new map. There will be two eras, which I think will be designated the High and Late eras. I'm running the map through LMM and I've got a dilemma. I was worried that if I used LandReg_0x ID's rather than the normal ones (ID_SCOTLAND, etc) it would crash the game looking for hardcoded events for High and Late (Magna Carta, etc) in provinces whose names it couldn't recognise, as it did on another thing I tried a good while back. I thought this might only work in Early, since VI unhardcoded it. Yet I notice the Hellenic mod has used ID_LANDREG as a faction identifier for ALL eras, not just early. Does it experience any Campaign map crashes? Does it do anything special to make this possible? Is it because they used completely different dates? If I used Medieval era dates would it crash say in 1215 because of the Magna Carta? Lots of questions that need answering to!