Resolved! Posting the script here and hopefully it helps someone in the future. Item Location Configuration is a separate record type to item.
function updReorderPoint(rec_type, rec_id)
{
var itemRec = nlapiLoadRecord(rec_type, rec_id)
itemRec.setFieldValue(‘reorderpoint’, null);
itemRec.setFieldValue(‘preferredstocklevel’, null);
nlapiSubmitRecord(itemRec);
}