It appears that Int64 properties are not supported. If you create such property EPiServer automatically uses backing type 'Number' for it, which only supports Int32. When you try to create and save a page you will get an 'Invalid cast exception'.
Am I missing something or are they really not supported. And if so, will they be supported in the future?
There is no built in property that holds an Int64. However it should be fairly simple to create a custom property for it, in that case you should inherit PropertyString and not PropertyNumber (since propertynumber is stored in SQL in an int column and not a bigint column).