I don't think there is a trick that FreeviewNZ-EPG uses.
Once the XML file is in the right folder, Mediaportal TV Server checks regularly (once a minute I think) for an updated file and imports it.
You have to go into the TV Server Configuration and make sure the names are mapped correctly and the directory to find the file in is set correctly... as you did with FreeviewNZ-EPG but I believe it will just work after that. I forced an import in the TV Server config software to test it but don't think I'll have to again. I'll let you know tomorrow night when hopefully it will automatically have updated during the day.